Check the saleability

[Incrementis]

Just a possibility worth exploring.
At some point, a completely new item type/category could lead to such behavior.
Registering item type into a store may solve the issue.

[Incrementis]

Adding the correct item type as the purchase type to the specific STO file solves this problem:

The next step should be to check the WeiDU documentation to see if a corresponding function already exists.

[Incrementis]

There doesn't seem to be a feature that adds "store purchase." Furthermore, it doesn't seem to be trivial to select all STO files in specific categories.

It seems that a custom function is required.