[SDPhantom]

I didn't realize you're replacing the StackSplitFrame with your own to overcome the stack size limit. The main problem is the API doesn't like being given a number larger than the item's stack size and throws an internal error. One feature of the original StackSplitFrame is that it allows keybinds to pass through. However, this is done by calling a protected function, which will be tricky to preserve.

I believe the best course of action is to repurpose the original StackSplitFrame, but it will probably take heavy modifications to allow purchasing more than the stack limit. I'll see what I can come up with, but I'm not making any promises.