For K1, there is no invoice. You can wait until receive the email from the Consulate, then fill out the DS-160, pay the fee, and schedule the interview. You can also email the consulate for the status.
Since you're already divorced, you can file under 5 years rule. However, the big issue is when USCIS run a check and find out about that. In conclusion, you need to have a really good lawyer if you want to apply for citizenship.