More detail on the issue here: https://gitlab.com/WoofHybrid/itemshopgui/-/merge_requests/1

TL;DR
The plugin was considering armor slots and the off-hand slot when checking if the player had enough space to buy an item.

Special thanks to JacksonChen666 for finding and fixing this issue!

The "previous page button", "next page button" and "balance icon" were all using the "back buttons" position.

Updating is recommended.

More information on what was added and fixed can be found on the github release page (the download button should take you there.)

If you have any issues or requests, you can post about them on the github issues page.