Has anyone else been constantly finding these things? I remember when they were really rare and it was a big deal to find them. Now every time I turned around someone is dropping one lol

A few days ago I got like 4 of them and other days I was desperately waiting for one. I think this random event is one of the more common ones, like I get challenged by the Ice King at least once a day. Contrary to events from the Tempest or the Oracle. And I guess it always depends how many pages you visit, on the day I've got 4 invitations I was willing my wardrobe very actively. It probably added up to 4000 page views, which makes the ratio pretty low. But I do get an invitation daily, and I feel like it is more common than last month, at least to me. But it should be completely random.

if the price of the things in the open market is any indication, overall there's been less of them than the demand for them lately.

I think it's totally random. I do all the quests that do Quest Points, so this is almost always manages to get me at least one.