As a Roblox developer, it is currently too hard to upsell subscriptions without making an entirely custom UI.
The Merch Booth developer module has historically been a great and easy way to quickly curate a store without making a custom UI. Unfortunately, it does not yet support subscriptions.
Attempting to add an item that is a subscription will not work:
MerchBooth.addItemAsync("EXP-1337", Enum.InfoType.Subscription)
If Roblox is able to address this issue, it would improve my development experience because I can easily add and upsell subscriptions using the Merch Booth module.