I'd like to directly address the comment you made in one of the answer:
Do you recommend I at least push back so he doesn't think he can treat me like a door mat? For example in our next meeting I can ask which inconsistencies caused the delay in payment.
Think about what happens if you don't pay your utilities bill. Will the utilities company call you, complain that you treat them like a door mat and ask why exactly you can't pay? No, they will send you a payment reminder, then another one with a late fee, and then they will hand the issue over to their lawyers. Somewhere in between, they will stop providing services to you. Nobody in the utility company gets angry, it's all part of doing business.
I know it's hard, but you also need to get rid of your emotions in this regard. You don't care what caused the delay in payment. That's, frankly, none of your business. Your contract says that bills must be payed within X days, otherwise a late fee of Y and Z% interest applies. If the customer pays within those X days, fine, otherwise you send the payment reminder with the late fee. If the customer then calls and complains, you might waive the fee if you like the customer and it was an honest mistake, but that's your decision.
Now, it's possible that your contract does not state these things. If it doesn't, that customer has taught you a valuable lesson. Be sure to fix this when the contract is up for renewal.