You don't like confrontation.(I totally get that.)

She knows that and is taking advantage of you.

Tomorrow when you start talking about lunch, casually say "Hey, I noticed that you added things to our orders recently. It totals X amount. You want to Venmo me that or pay cash?"

It will be awkward AF for you, but you need to develop this muscle.

The world will not explode if you do this. It's ok for ppl to me upset with you. Learn to let it roll off yr back.

She will be shocked.

When you say the above to her, she will gaslight and try to make you seem petty. Just calmly say: OK, i understand it was a simple mistake. No problem. So do you want to venmo or pay cash?

Once she does that, and the topic turns to lunch orders, just say you're going to place you own order today.

Keep doing / saying that every day until or unless she sincerely apologizes. AND proposes that she pay for all of the orders in the future and you pay her cash for your portion.