How to Deal with the 3 cm Thickness Limit and What to Use Instead
Have you ever finished packing a product, measured the thickness, and found yourself staring at 2.9 cm, only to discover later that the shipment was returned as oversized?
That final millimeter matters more than many merchants expect. Over more than 2,200 shipments, I have hit this exact problem many times. The 3 cm thickness limit is one of the most common real-world constraints for stores using small-parcel shipping methods.
In this article, I will explain both how to pack more efficiently to stay within the limit and what realistic alternatives to use when you go over it.
Why the 3 cm Limit Feels Like a Wall
Many small-parcel services share the same limit
Click Post is not the only service with this rule. Yu-Packet, Kuroneko Yu-Packet, and Letter Pack Light also rely on a 3 cm thickness ceiling. Once you exceed 3 cm, your available options narrow very quickly.
For reference, Click Post accepts packages up to 34 cm x 25 cm x 3 cm and 1 kg. That means you need to think not only about thickness, but about the full packed size.
Packed thickness is not the same as product thickness
A product that is only 2 cm on its own can end up at 2.5 to 3.5 cm after bubble wrap and an envelope are added. In practice, packaging often adds 5 mm to 1 cm.
That is why the final measurement must always be taken after packing.
Packing Adjustments That Help Stay Within 3 cm
1. Reconsider the cushioning material
Standard bubble wrap is familiar, but it adds thickness quickly. In some cases, thinner materials can give you enough extra room.
| Cushioning type | Approximate thickness | Characteristic |
|---|---|---|
| Standard bubble wrap | About 3 mm | Common, but bulky |
| Slim air cushioning | About 1.5 to 2 mm | Thinner, slightly less protective |
| Tissue-style wrapping | About 0.5 mm | Good for scratch prevention, not strong impact protection |
| Foam sheet | About 1 mm | Thin but fairly protective for small items |
For resin keycaps, I often prefer foam sheet because it saves 1 to 2 mm compared with bubble wrap. That said, the right choice depends on how fragile the product is.
2. Use thinner envelopes or outer packaging
Rigid cardboard envelopes and padded mailers are convenient, but they add thickness on their own. Switching to a thinner kraft envelope or OPP-based outer layer can create a bit more room.
The tradeoff is durability, so this only works if the product itself is protected well enough.
3. Change how multiple items are arranged
If multiple items are stacked, thickness adds up immediately. In some cases, arranging them side by side rather than on top of each other solves the problem without changing the shipping method.
4. Use a real thickness gauge
Guessing leads to returns. It is worth keeping a 3 cm slot gauge, caliper, or similar measuring tool near your packing area so you can check quickly every time.
What to Use When You Exceed 3 cm
If you cannot realistically stay within the limit, change the shipping method. Forcing the package into an unsuitable service increases the risk of both returns and product damage.
Letter Pack Plus, 520 yen
This is often the first practical step up from Click Post.
| Item | Details |
|---|---|
| Price | Flat 520 yen nationwide |
| Size | A4 envelope size |
| Thickness | No fixed limit as long as the envelope closes |
| Weight | Up to 4 kg |
| Tracking | Yes |
| Delivery style | Face-to-face delivery |
Its biggest advantage is that thickness is not limited the way Click Post is. If the item still fits the official envelope and the seal closes properly, it can be shipped.
Yu-Pack
If the item is too large even for Letter Pack Plus, Yu-Pack is the next likely candidate. It costs much more than Click Post, but it also adds compensation and broader handling flexibility.
Yamato courier service
This is a similar alternative to Yu-Pack. It can be convenient if convenience-store or Yamato-office drop-off fits your routine better.
A Simple Decision Flow
After packing the item, use this logic:
- Is the packed item within 3 cm, 34 cm x 25 cm, and 1 kg?
- If yes, use Click Post
- If no, continue
- Does it fit in Letter Pack Plus and stay within 4 kg?
- If yes, use Letter Pack Plus
- If no, continue
- Does it fit into the size range for Yu-Pack or standard courier service?
Putting this kind of flow on paper near your packing area makes the decision much faster in daily operations.
Using Different Shipping Methods for Different Products
Not every product needs the same shipping method. In many shops, the best approach is to divide by product type.
For example:
- TRRS cables and single keycaps: Click Post
- Keycap sets or thicker combinations: Click Post or Letter Pack Plus depending on final thickness
- Larger keyboard parts: Yu-Pack
In Shopify, you can reflect these differences through shipping settings so the correct option appears based on the contents of the cart. For a broader comparison of shipping methods, see our comparison of small-parcel shipping services.
Common Questions
Q. What happens if a mailbox shipment is actually over 3 cm?
If it does not fit the slot or fails the size check, it may be returned or held rather than delivered. That means extra time, rework, and a second shipping attempt.
Q. Can Letter Pack Plus envelopes be reused?
No. They are intended for one-time use.
Q. How much safety margin should I leave if I am close to 3 cm?
It is safer to leave a few millimeters of room rather than aiming for exactly 3.0 cm. Compression and slight shape changes during handling can push a borderline package over the limit.
Q. If a Click Post shipment is returned for excess thickness, do I lose the original shipping fee?
You usually need to create a new shipment and pay again when reshipping, so avoiding the error in advance is far better.
Conclusion
The 3 cm thickness limit is often manageable with small packing adjustments, such as changing cushioning materials, using thinner envelopes, and arranging products more efficiently.
But forcing a package into Click Post when it clearly does not fit is the wrong place to save money. Once it exceeds the limit, switching to a service such as Letter Pack Plus is the safer and more professional option.
Next to Read: Articles Around the 3 cm Constraint
Handling the 3 cm wall is most efficient when you look at packing, shipping method, and fulfillment flow together.
- Go deeper on packing technique → Packing Basics for Click Post to Prevent Breakage and Rain Damage (covering the no-insurance risk with smarter packing)
- Rethink the shipping service itself → Click Post vs Yu-Packet vs Nekopos (size, price, and tracking tradeoffs)
- What kinds of products belong on Click Post? → What Kinds of Products Are a Good Fit for Click Post
- Organize the full Shopify → Click Post workflow → Shopify x Click Post Shipping Guide
Pair the thickness fix with CSV export and auto tracking sync
For products that do fit within Click Post's limits, combining Shopify order import (CSV export) with automatic tracking-number sync after shipment can compress the before-and-after desk work into a single pass.
→ Auto-Sync Click Post Tracking Numbers to Shopify (turn 50 minutes of typing into 10 seconds for 100 orders)
The Shopify app Instant Shipping! for Click Post handles everything from CSV export to tracking-number sync in a single click. Reclaim the time you spend worrying about the 3 cm limit by tightening the workflow around it.
