As you might know, we had issues with people buying shulker boxes from the shop. The money would be taken but they won’t arrive in the inventory.
I tried to identify the issue but could not replicate it. So I changed the way transactions are logged in the system and made it log the full command that is used to give items to users.
Then, VixenGold had the same issue again and kindly reported it to me.

It turns out that the issue only comes when you try to buy more than one at the same time. The system refuses to hand over 2 items at once. That is why it worked for some (they bought only one) and not others (who tried to buy more).

It turns out I had a wrong asset-config where I store how items stack. Shulker boxes were registered there as items that are allowed to stack 64 items, in reality it cannot stack at all. Assec config fixed, problem solved.