For me I tend to tuck in some small art prints, maybe stickers, post notes, various ephemera. As a Uk swapper though its all about keeping that postage under 20g or 50g depending on the swap.
I think its also important to say that printing out things you've found on the internet is a bit of a dodgey practice unless you have permission from the artist whose work it is. There are some great places to buy extras if you are stuck.
Shipping heavier items can result in high costs, but sending flat items such as stickers and ephemera through flat mail is an excellent alternative indeed!
Did you know Snail Mail Friends swap allows you to set a weight limit for your package? This might be a helpful option to use when making a swap 🤗
It is crucial to give credit where it's due and obtain permission before using someone else's work. Even if my life depended on it, I couldn't draw to save myself! 🤣 There are indeed, as you mention, great places to buy extras, Creative Fabrica is an affordable option that occasionally offers fantastic discounts.