@STMN: that is correct, you get only one star, no matter how many cards you buy in this offer
Yeah - I fell into this trap as well - just bought 24 cards and only got 1 star. I ***** exclusive cards in general and would never buy a card I didn't want in order to get an exclusive card no matter how appealing the card. But now that there is zero chance I'll get the exclusive card I will be very annoyed.
The secret to this offer (or rotation of the offers) is to spend slowly if you want the exclusive card. It's not about how many cards you buy or how many credits you spend. It is about how many offers you participate in so if you like an offer just but one and wait for it to come back then buy again.
I don't have the time or interest to monitor the offers hourly but I'll check back Sunday and see if any offer I might be interested is worth 4 stars and hope I can find 3 of them - not likely.
On a side note - I rushed to buy the 24 cards before time ran out. In fact I bought 20 before time ran out and when time went to zero and I got my SINGLE star the time magically went back to 20 something minutes? So I bought another 4 cards and that went to zero with no addition star. But then the clock went to 4:17 and stayed there for 20+ minutes and has now been stuck at 00:14 for some time.