Definitely not pay2win. You could pay for faster progress but that was mostly cosmetic. Everyone got e.g. the Berry Shooter Pea fully upgraded for free, or the Dr. Chester on the zombie side. Those upgrades were somewhat usefull, but it's not like you couldn't play with a non fully upgraded class at all. Also, like tempest wrote, you earned quite a lot of coins by just playing the game. 10K and more for a round of Gardens & Graveyards more often than less, lasting around 15-20 minutes. The prices are 20K for the best mixed upgrade pack, 40K for a guaranteed character variant.
Since I got the game for free, I put around 10 in for coins packs just for the joy of it.