No, you have to sort of figure out what the average cost is and come up with a reasonable flat rate.
If this is for Canadian orders, one thing to consider is that Canada Post has Flat Rate boxes in different sizes with prices ranging from $17-$32. So that at least gives you a worst case scenario type ceiling for what shipping might cost if your Stallion flat rate turns out to be too low.
For USA shipping, you might want to consider printing labels directly on eBay. I don't know what the current Stallion rates are, but eBay's Tracked Packet discount is about the same price as sending with Chit Chats. With the benefit being you can simply drop everything that fits into a Canada Post mailbox. If you do this, you can use a shipping calculator. To show buyers the eBay discounted rates, you have to visiting shipping preferences in your account settings and check the box to show discounted calculated rates.
