Very impressive game, nicely done.
I noticed a couple of issues on a somewhat lengthy endless run:
1. Inactive pumpkin
Once you have 4 pumpkins in potted plants, 100% of armor is retained each round. However if you get a 5th, then realise it does nothing and remove it again, you end up with 4 but this time only 75% of armor is retained. And if you remove another to have 3, you keep only 50% where it should be 75%.
Flower pots do the same thing also with number of cards instead of armor retention.
2. Infinite armor
If you buy 4 magnet shrooms you steal 120% of zombie armor. This interacts strangely with the Z-Mech zombie who applies armor to himself when you gain armor. In a fight with the Mech zombie if either of you apply armor you both instantly gain NaN armor and it's impossible for anyone to die. The only way to continue the game here is to restart until you get a different opponent zombie.
3. Flower pots
They seem to stop working at around 8 cards and after that getting more flower pots does nothing, but you can still waste your money buying them anyway.
4. Marigold
The description says "For every 50 coins, it costs one less sun", however it seems to always cost 3 suns regardless of coins. Maybe this is not a bug and I simply don't understand what it means.
There were also some times where my armor would suddenly shoot up to 999, but I don't have any clue what it's related to, something about the magnet shroom as well I think.