Does not work. The chrome version does add items successfully. Tested in Booth shop.

Shitty service, shitty extension, it reads much more of your info than it needs to and due to some errors can lead to cancelled orders (because of doesn't account for shipping price or something, who knows) and fees spent for nothing. The service itself takes ridiculous amount of fees and they don't do much to earn them - it's hard to reach them (takes 30mins-1hour of waiting always), they aren't here when you need them (like on weekends, when your order is cancelled somehow and you need to buy the thing because otherwise it'll be sold out).
Avoid.