I think the faq needs a clarification for users and developers.
https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced...
https://itch.io/docs/creators/download-keys
How is the associated price of a download key generated (or compared to individual priced files), when the key is part of a bundle or at a discount or at a discounted bundle or at a discount in a discounted bundle?
The faq could also point out, that it is better to create extra projects if one has extra content to sell and point out the drawbacks more prominently. Like locking the price, or else lock out previous buyers. Which happened for OP's customers. I could imagine a suggestion to consider creating a bundle of base game and extra content, instead of price tiers. The biggest problem with the price tiers is, that you cannot increase your tier.