To pay use tax for Amazon purchases: should I calculate the value of purchase after applying gift cards & pts or before? i.e. $39 vs $14 after applying $25 gift card/pts

The value of purchases must be taken before the application of gift cards. Gift cards are only a payment tool.

Thank you! In case of coupons / vouchers - those are considered item discounts, hence I should consider the discounted price of an item, correct?

Yes, you are correct.

Got it, thanks for your help!