Hello.
At the moment when you upload files, and get a price, you can't change it later by yourself. The one way now is to write to us, that we could "reload" your product and that system would take newly uploaded files and calculate prices based on new dimensions.
We are still fixing this thing so that prices would be shown correctly and one big fix will be live soon which, as we guess, should fix all the process later.
Hope that's clear now.