I don't know how bundles work, but if you have a page with a pack and you add files to that pack, the additional files are considered part of the same pack and can be downloaded by anyone who has paid for your pack.
One solution is to have different prices within the pack, these work based on a tier system.
For example, if you have 2 files in the same pack, one for $5 and the other for $10.
"A" buys the product for $5, he will only be able to download the first file and not the one that costs $10.
"B" pays you $10, he will be able to download the 2 files.
Tomorrow you add 2 new files, one for $8 and one for $12.
"A" won't be able to download any of the new files, because he can only access the ones that cost $5 or less.
"B" will be able to download the $8 file because its value is less than the $10 he had already paid, but he will not be able to access the $12 file unless he pays again.
Important, "B" must pay $12 again, Itch is not cumulative, that is, if you already paid $10, paying $2 is not enough, you must have a single payment of $12 to be able to access that file, regardless of previous payments.
From the point of view of the consumer, it is a bad practice to place files at a higher price within an already sold pack, many people (including me) see that as an abuse and may not buy anything from you again. But it's something you can do inside Itch.