In retail, I'm spending Bullion to unlock some weapon transmogs, and this pop up shows for each item.

There doesn't seem to be a checkbox for it in the addons options.

Can you please give me the name of one example item that gives this popup?

Oh, I just noticed you said bullion. Let me look into it, but since Remix is about to end, I don't know if I'll be able to update it in time.

Bullion is a Season 4 rewards from raid bosses, but this error pops up using the transmog unlocks

Here are a few:
Crucible Raider's Bronzed Mutilator
Crucible Raider's Bronzed Wingblades
Vault Raider's Crystal Fists
Vault Raider's Crystal Scepter

This is fixed in v23.