Hello Henry,
This isn't possible to do today unfortunately. The minimum quantity that you can set for each line item is 1 and not 0. The validation in the API predates the addition of the `adjustable_quantity` parameter/feature so maybe we could change this in the future.
In a way, you make your PaymentLink and the resulting Checkout Session(s) kind of a cart system which we definitely didn't intend this for. I can see how your use-case could be interesting so we'll track this as a potential feature request.
In the meantime you will either have to default to 1 and let them change it as they want or maybe integrate directly with Checkout and ask them upfront which Price and quantity they want to pick.
Sorry I don't have a better answer for now!
All the best,
Thomas