Couldn't you just remove the flat postage rule on the Postage Preferences page?
Only way I can find that page though is to go to help, seach postage discounts and the wording for that page is highlighted. Someone else might be able to give directions to find it from the seller hub.
Presume though that you have not opted into postage policies, which might be a good option for you - that way you can make any number of postage policies, and when you want to change anything, you just make the changes in each policy - and you can check at all times which listings have which postage option. I like this option on our business account with 1800 listings, works well for a large number of listings.
