I really like the concept and I think it could help kids get practice counting coins.
The coins spat out seem to keep a high speed making it sometimes get in the way, the purple goo making things disappear was also causing similar frustration? There were times when I wanted denominations that weren't there (not sure if that is what you want) like I needed a quarter but had to make do with nickels and dimes. At times I tried to click on things and other items got in the way or just didn't act as I expected, sometimes coins would appear in the way of others?
Idk what system you are using but you could do a calculation for the total needed, probably using a mod function (14 mod 5 would give you 4) the coins could perhaps be tracked in a list and giving the player a couple extra of the ones they need. maybe I just had a fluke on a level or 2
Rather than spawning the coins in the box you could have them fall or slide into the box, if its unity you could temporarily disable its rigid body/ box collider and change its position without physics and re-Enable it with like a coroutine after it presumably got in the box or it could even wait until its position is within the bounds of the box (maybe calculating for the different sizes of coins)
all and all I think this is a pretty good project, just my thoughts though I don't know what your design goals were