I don't think you need to send additional proof unles they have asked for that.
If you really want to send additional proofs then your letter should contain all the information they need to match it to your file. Including your full name, file number, and UCI number. Otherwise they may have difficulty gettiung it to the right VO and the proofs might get lost.
Other than that there is no specific requirements of the letter.
Just say something like:
"To whom it may concern,
I would like to include additional "proofs of relationship" in my application for spousal sposnorship. These include photos of our recent visit together, tickets, and shopping receipts. Please attach these to my file, which is File No:9999999
The photos are of our trip to the calgary stampede
The tickets were for the calgary stampede
The receipts are for matching cowboy hats.
Thank you for your consideration of these documents"
Or something like that will probably be fine.