I've never understood the appeal of using a product like Affirm, especially if you already have a credit card. Credit cards already give you a 30-day grace period before purchases start accruing interest. Using Affirm to extend that to 42 days seems inconsequential. It seems like their target customer are people bad with credit, which would make them predatory.
This depends on the timing of your transaction and your statement date. If you purchase something 1 day before your statement date, and you don't pay it off that same day, you will accrue interest.