You can buy blank Avery address labels from staples or office max in different sizes and shapes. Then go online to Avery.com and use their templates to create your return address labels but you’d have to enter in the address one by one for your addressing labels. I’d print two of each so you have them come invitation time too. You can even add pictures, logos, etc. just an idea, it’s most likely what I’m going to do.
I printed directly on the envelope. I used pages but you can use any word program. Choose a template, correct size envelope, and then type whatever you want. I put the envelopes in my printer and just sent the paper size on the printer. It took a few trial envelopes to make sure the placement was correct.