Spoiled much?
If you're miffed about the amount of money you made, that's on you. Mark up your prices more if you want more.
As far as waiting two months, every place I've worked pays you for the previous month's work. Tack on FAA's 30-day return policy and you have a two month waiting time.
It makes complete sense to me. I see no reason to be impatient. Use the time to make more artwork and promote yourself.
You should be happy you sold your images, not weeping over invoices.
Like April (with an exceptionally amazing name, I might add) said, it's all about perspective.