How to use Sezzle Spend on Amazon: Sezzle's own steps, read
As read, Sezzle's Amazon page never names Spend. Its help article says where the toggle sits, which instalment goes first and which orders skip it.
Subscriptions
Sezzle Premium vs Sezzle Anywhere: what each tier adds
Sezzle Premium vs Sezzle Anywhere, from Sezzle's own pages: where each lets you pay later, which Service Fees go, billing rules; no price on pages read.