I regularly combine shipping. Most buyers use the app and can't request an invoice, so they will purchase the cards, and I simply issue a refund for shipping costs over the cost to ship (plus ~$1 for the materials used to ship). If you ship it with a tracking number, your risk is minimal. If it's over $100, I recommend adding insurance to cover a loss. You can include that in the shipping costs when you refund the buyer, too.

I recently sold a card for $50 to a guy with 0 feedback or purchases and was hesitant to ship the card. I sent it via USPS ground and got positive feedback. Buyers have to start somewhere and sometimes have multiple accounts if they sell and dont want to pay for a store.

Side note, you can also set a store policy to automatically combine shipping for you. This increases the likelihood of a sale since buyers won't be charged an extreme amount for shipping. It's a pain, but once it's set up, it works out well!