[mothered]

Is there a purchase price limit for the DNA method ?

There are no hard and fast rules, but do use common sense and good judgement.

As long as you're not SEing a sports car (so to speak), the DNA Is not capped.

What happens if I buy a product that costs 15 thousand dollars, which is just delivered without OTP?

Extremely unlikely, If not Impossible for an Item of that value to be delivered without a secure form of authentication.

And is the seller (amazon) or the supplier liable in this example?

Generally speaking, the supplier will foot the bill and Amazon will reimburse the funds.