In addition to the @store_id, there should also be a @company_id

[juuso]

Currently, the gem uses the @store_id variable to build all URLs, even though most of the API URLs actually require the company identifier, not the store identifier. For many companies, they simply happen to be identical by default. This is not always the case, though.

This bug quickly becomes apparent when you have more than one store under your company account.

Here's an example URL where @store_id is used correctly:
https://sites.fastspring.com/#{@store_id}/...

And an example where it's used incorrectly:
https://api.fastspring.com/company/#{@store_id}/...

[juuso]

Added a pull request.