[qualiteam]

What if a customer opens such a link but with "price=0.01". Will it allow him to buy the product for a cent?
It looks very unsecure to me.

I think you should look into using REST API to inject one-time validation codes into a custom XC5 table, then pass a hash from this code along with your "custom price" request and validate if the hash compares to the validation code stored in the database. But this requires a custom module, of course.