IMO, the primary use for instant downloads will be e-cards/cards shared on socials and sent via email or messaging. That’s also what we want customers to do - not buy one download and print 200 invitations. And that’s a product I want to offer separately, designed and optimized for that use.
In these cases, you usually send/share/publish just one image that includes all the content (image and all text) at an appropriate size depending on platform and purpose. The ideal case would of course be a dedicated product, one-sided, at the most common sizes and ratios for sharing purposes, and for digital purchase only. But this is probably asking too much. So the suggestion is to allow us to set instant downloads as the default option for designs we want to be used primarily for that matter (I would choose flat cards - content on the front and the back either blank or a pattern with logo, whatever).
Just an idea.