I'm glad you like the washi. Im happy to trade samples any time since I have too many! I usually get my washi tape as a set off amazon (although its a little pricey) and Mercari :) the brand off Amazon is from "infeel.me". You can buy the tapes individually off of aliexpress. I don't know how to rename links 😅but I think this is what I used on your envelope.
Here is a link to the Mercari seller I buy from. She's lovely and usually ships the same day. She has a great selection of really unique washi! All my favorite washi tape are from her shop.