Hi Salva
Indeed I have purchased it for my live site (1 license) but do not have 2-3 extra copies for my dev, staging etc.
If you're confident that the ATUM is not preventing a user from completing a checkout I can have a closer look to see the error. I only noticed it once the plugin was activated and setup (and copying everything from live to staging) that I noticed Atum wasn't allowing me to proceed
So to confirm you said because I have a copy on my live environment, I can use the same key on staging and dev?