Mees-Gray-Eagle What you propose does work, but then you can simply adjust the amount on the site.
Even if that is not possible on the page itself, people can adjust the URL.
I also understand that bunq does not issue links that will continue to work forever. That is nice for now, but in 3 years you will have a huge amount of links that will probably never be used again but you have to keep.
How many webshops are there anyway who only sell a few items, never have someone ordered several items or different items and have no offers?
Moreover, they apparently also process the payments manually because there is no link with the API and they also do not use any other party like Mollie. Then creating those few payment requests once a month does not seem such a problem to me.