I am still hearing from friends and family who are receiving my Christmas cards now (third week of February). And I know some of my Valentines have still not arrived (like the one I mailed to my husband who lives in the same house as me!). The US mail has been extremely backed up ever since the holidays but seem to be working through it. I had three cards returned to me that were written in very clear Engrosser's Script with no flourishing. All three had the correct address. Just said undeliverable.
A friend had a card sent to me returned to her and it had two stickies over it. The first one had my correct forwarding address on it. The second one said, "Return to Sender; Insufficient Address; Unable to Forward." For no apparent reason. She put it in a new envelope and mailed it again with the new address and it was finally delivered. (See photo.)

Given that, I am trying to be conscientious about not making the postal workers' jobs even harder by trying to decipher addresses through all my flourishes.

So I put my Valentine exchanges inside a clear envelope with a white piece of paper over the top with a text printed address on it. It's not as exciting to receive but at least it gets there! (See photos of my Valentine exchange below.)
Schin Loong suggests using a craft blade to cut out a rectangle over the stamp area and put the stamp on the original envelope. This way there is a postmark on the envelope itself instead of the clear one. Either way, so far folks are receiving them.
Clear Bags (Envelopes) is having a 10% off sale right now using "SAVINGS" as the code. I plan to stock up.