Anyone sold at an actual store?

I have a yarn store carrying my stitch markers. She didn't give me a price she wanted to buy them at, just saod to invoice her but I don't know what to charge her. Do I ask for a percentage of sales (like 30%), do I make it a flat rate ($6 for the $10 sets and $10 for the $14 sets) or do a combined percentage or flat rate depending on which is more?