Are 2 $5 options stackable to purchase a $10 option?

nope. because you can only put in one code at a time, and as such it takes it as 2 different transaction!

hmm interesting. ill admit i have never done getting RP like that. but i was under the assumption that it was like buying regular RP. But seeing how you can add more than 1 card at a time, it should work assuming you put the option of getting the 10$ worth of RP. it wouldnt hurt to try!