Great game! Only three real points of contention:
1) After you put a letter/parcel in the mailer, it takes a delay to work on the next one. Many times I clicked the mailbag only to be told to finish the current letter, even though it has already been sent.
2) Mechanically, there isn't really a reason to buy any of the repair tools. It is faster to just put them in the undeliverable pile and move on, whereas the desk upgrades make everything easier and award you a higher salary. Obviously it's the most fun to fix them, but it's hard not to feel like you're throwing money into a hole unless they're cheap... which occurs after you buy the desks instead.
3) Considering how often you have to use the portal, it takes too long. If the mailbag was ALSO present in the moss dimension, it wouldn't be so bad. Work on the mail until you get one in the other dimension, swap, work on the mail until you need to swap again. Either that or speed up the portal time, it's bothersome to wait every time you need to go to the other place, and then back, and forth, and back, and forth...
3.5) I'm sad I can't dig the undeliverable mail back out once I had the tools. It's undeliverable forever!!
3.75) Edit: I forgot one! When the auto-sorter is purchased, you can click a letter and put it in the now-equally-colored sorter, implying you can put them anywhere. This is not the case and I was deducted 100 stamps at the end of the day. Then I realized you right click the letter and it auto sorts/sends. Both should probably work, or disable the option to manually put them in the sorter.