I believe double-clicking on the item in the shop will bring up a window where you can input the number you want to buy.

I tried it and it didn't work. It just selects it and deselects it. I've also tried shift clicking and ctrl clicking. Alt clicking doesn't do anything, like it wont even select the item. Maybe I'm thinking of BG2 or a different title entirely.

The BG1 engine won't allow you to do that, unfortunately. The BG2 engine does though.

Just tried out the BG1 vanilla game again yesterday after years of Tutu and I found the same thing happening as well. It's one of the reasons Tutu is easier, along with the Tab-highlighting and pause-at-inventory, it's a good top 3 of reasons BG1 vanilla feels old. But I'll try it for nostalgia's sake.

For those new to the game: Tutu=BG1 in BG2 engine.

To be honest Dub the thing of not being able to put pause in the menu at battles is one of the things i love from baldurs gate vanilla , ok it might not be very handy sometimes , but think about it , its not like in the middle of a fight you can tell the monster , hei way i need to get something from my bag xD , so it makes the game a bit harder sometimes , a real pain another times , but for me its one of the little things i like about it :)