I seem to get up to 5 a day, depending on how many pages I visit, but the prices for those things suggest they're a bit rarer...or are they?

The prices tend to fluctuate depending on which items are in the shop. Usually when there's a new set/new retirements the prices for the invites go up. When things kinda slow down the invites kinda drop a little.

Wow , either you are very lucky or I'm just very unlucky - I've only found 1 since I joined! 😮