Great update, thank you for adding this. This exact scenario happened to me last night.
I went to look for deals on GF legendary reward cards and found a fully maxed Oshannus for $90. I put the card in my cart and went to look for another $10 worth of cards to receive the $100 PKM bonus and ofc in the 2 mins it took me to find another $10 of value the Oshannus had been purchased.