I bought some items as a Guest, before deciding to Register. I now would like to give feedback for the items I have bought, but I can't, as they don't figure in my account.
Feedback is much less important than it used to be.
Use Contact Seller to let your sellers know that you received your purchases and are pleased with them. At the same time you can let them know your shiny new user name.
I had a buyer who purchased as a guest and, instead of leaving feedback, he did as femmefan1946 suggested and sent me a nice e-mail letting me know he received the item and thanking me. It meant more to me on a personal level than an 80-character-limited line of feedback. Your seller may agree.