Regarding the design, I think it's already very good, and I personally like it. However, I suggest adding a barcode area in the lower-left corner or another location to prevent regular mail barcodes from obscuring important information. As for the postal code field, I don't see any major issues. Although I primarily based my design on relevant standards, as suggested by the previous commenter, postal codes are not widely used for sorting in most areas other than Beijing and a few others. After all, the recipient's information is already entered into the receiving agency's system; scanning the barcode is sufficient.
As a fan of postcards, I have also designed the card in the format of a postcard. Here is my own attempt to get the ball rolling:
QSL card made before receiving the callsign ↓
After receiving the callsign, printed QSL cards (vertical format) ↓
The above cards were sent using postcard-style delivery. After receiving confirmation from the corresponding station, they were successfully delivered and showed no obvious damage. (If you also have similar postcard-style QSL cards, please feel free to contact me to exchange them.)
Also, let me show you the custom-made "call sign" labels that can be used for different dates.
Although I didn't exchange my card very often, I did exchange it a few times.
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