SnapCorrect are willing to accept impression kits from other aligner providers, or from your dentist, if you’ve decided to make the switch to SnapCorrect for your aligner treatment. SnapCorrect can even provide you with a shipping label so you don’t need to spend extra to get your impressions to them.
Just make sure your impressions are carefully packed so that they aren’t damaged in transit, otherwise, they won’t be usable and you’d have to buy a new impression kit to start your treatment process again.