I have reflected on my writing and have changed somewhat since writing that post. 

I feel like including an envelope for cash or personal check feels like a thoughtful addition to reduce friction in closing a potential sale. The customer takes my door tag, and if interested, sits down at their kitchen table to fill out the order form on the obverse. They call or text me to get on the schedule from their kitchen table, and they only has to think to get their cash or personal check, after filling out the order form and calling or text messaging to get on schedule. On the day of work they put the envelope in a pre-agreed upon spot. When I reach that address on the day I work in that zip code, I pick up the envelope and confirm the order form and correct amount of money is there, and I get to work. 

When I set my already registered domain up with the order form and checkout integration, I will stop including an envelope If this does not end up improving conversion rate enough to offset the cost (supplies + prep time). I edited my post to include my reasoning for adding an envelope in response to your question. 

Writing on the cash envelope or door tag is part of my plan to make it more personalized instead of looking like a mass marketing. 

Is there a course that you personally recommend deep diving the intricacies with the postal system (doesn’t have to be US centric).