itch.io

Hi, we're not doing the payment processing ourselves, Itch handles everything regarding payments.

I will contact them to see what's the problem with your payments and I'll come back to you in the upcoming days with their answer

I have reached out itch.io's support team and they have confirmed that payments are NOT cumulative. You have to pay the minimum amount at once. 

Itch support initiated the refund for your transactions ( you should have them back in the upcoming days from my understanding ) then you can pay the whole amount at once if you want