Hello!
It’s pleasant to see that new forums for writers appear in the Internet. Indeed that’s very helpful and useful to communicate with people who share something in common, like writing

As for your question, from my experience I would strongly recommend you to agree on price before submitting any of your articles. I understand your embarrassment, but are you in this business because of philanthropy?

Your articles are the fruit of your work. If you don’t appreciate it and if you don’t demonstrate that you appreciate it, hardly anyone will do that for you. Please don’t think that I am too pushy when writing such things. I remember being modest and uneasy when talking of payments, etc. as well. But that’s not the best way when in writing business.
Good luck!
Hope to hear you here.