jeremylautman
Lol @rogercbryan. Well said.
Talking about compensation is scary. Believe me you aren't the first to have this problem. When I assembled my design team for my product, we never discussed how much each of us owns. Now, as I'm trying to start a company around the technology, I'm being forced to overpay considerably for the technology. No fun.
The fact is we avoid things which make us feel uncomfortable. This is true in topics ranging from money to religion to dating and to relationships. The fact is that the longer you avoid facing that which makes you uncomfortable, the worse it gets.
I don't think I agree with JustCallMeRoc when he says that you don't believe you deserve the money. While that's one possibility and could be the case, I think it's more likely that you, like most people, avoid topics which make you uncomfortable. The only thing you can do is bite the bullet and do it. One way to do this is to set a written rule for yourself to get the invoice out the moment you send the work, and post this rule above your desk.
Over time you'll get used to it and it won't bother you anymore. It's the same thing with guys who have trouble talking with women. They feel nervous so they sit in the corner of a bar all night praying for, yet dreading at the same time that a girl comes over. However, if you put yourself out there enough times, you learn that it isn't as bad as you thought. Don't delay. Send those invoices out the day you finish work. After a few weeks it won't bother you anymore.