Check out Postal Arbitrage by Riley Walz 
A stamp for a letter costs $0.73 in the United States. Amazon Prime sells items for less than that… with free shipping! Why send a postcard when you can send actual stuff?
I found all items under $0.73 with free Prime shipping — screws, cans, pasta, whatever. Add a free gift note. It arrives in 1 or 2 days. Done.
You’re not only saving money. It’s about sending something real. Your friend gets a random can of tomato sauce with your birthday note attached. They laugh. They remember you. They might even use it!
Then there’s a list of products on Amazon, no affiliate links.
Why not build your own version of this site with affiliate links? 
Sure, you’ll get tiny commissions on such low-priced items, but you also earn commissions on ALL items someone buys within 24 hours of clicking your link.
Once built, the site could run on autopilot 
Over time, develop it into a more comprehensive gift ideas site, like these…