What are these A2 and A3 stickers?
While it's true that this manga lacks any dialogs, there are actually 2 pages at the end (pg. 196-197) that describe the whole scenes from the beginning until the end.
For page 30-31,


30-31 鞄の中にファクシミリ用感熱紙、コピー用紙、ファイルブックなどの文具又は書類が覗いている。
30-31 Thermal papers for facsimile, copy paper, filing book and other stationaries or documents are peeking through the bag.

Researching those things returned these images:
 
Thermal papers for facsimile by Fujifilm | Copy papers by Super Echo
Thus, I believe spiel's comment was actually right: they are standard paper size labels. The A3 thermal papers are rolled into a cylinder, and the A2 copy papers are showing their shorter side. (Which means that the filing book is also A2-size, which also exists)