How do you do that trick?
OP's fidget spinner on
Prime Now.
Basically, you top up your Prime Now order with physical gift cards to reach the $35 free shipping level.
So if you only need something for ~$5, you also add a $35 GC. If you're items are ~$5 to ~$10, add a $30 GC etc.
Link to
physical GC (multiple denominations available).
You can't use GC's to buy more GC's (thereby perpetuating the cycle), so you'd be saving the GC's for future regular purchases.