My daughters and I ended their summer vacation with a clutter-busting kick that included cleaning out their toys, clothes and books. Then, in an effort to fund their allowances through Christmas–and not add to any landfills in getting rid of their unwanted items–we decided to put stuff up for sale on eBay.

Now, I consider myself to be a pretty savvy eBay veteran. I’ve been buying and selling items on eBay for years (actually ever since I wrote my book Buying and Selling Your Way to a Fabulous Wedding with eBay and became an eBay fanatic during my book research). One of the cardinal rules in making eBay work for you, financially, is making sure that you don’t stiff yourself on shipping. I learned this the hard way the first couple of times that I’d sold stuff on eBay and didn’t weigh the packages before I’d listed the shipping price. From then on I was super careful about shipping and making sure that I always turned a profit.

Then the U.S. Postal Service had to go and raise its rates, and create this odd rubric for figuring out postage for anything that isn’t a letter. It’s all so confusing and frustrating that I can’t ever determine shipping at home anymore and therefore have to schlep to the post office now whenever I’ve got something over 16 ounces I need to mail. I hate going to the post office.

Last week I noticed that on the back of my grocery shopping receipt, there was a $1 off coupon for my local shipping store that uses brown trucks exclusively. (You know which one I mean, right?) So having successfully sold three lots in the past two days, I figured I would avoid the post office and its inane postage rubic, and take my packages to the shipping store and, of course, use my coupon. Again, I figured, my packages weren’t big and they weren’t heavy, and so they shouldn’t cost too much to ship.

Wrong! I ended up spending $10 more on shipping than I’d made on the auction itself. I feel like such an idiot that I didn’t take the packages back once the guy had rung up the charges, but I think I was in shock. And so I silently walked out of the store and then began to beat myself up once I got to the car. I mean, I’m selling stuff on eBay to help our Sudddenly Frugal budget, and here I’ve just taken $10 out of the budget. That is not good budget math.

Turns out when you use private ground shipping services, the discounts come with size, weight and volume. So if I’d been shipping a big box that weighed a lot, it would have cost signficantly less than if I’d taken it to the post office. But my three itty-bitty padded envelopes of books and games? I really paid a premium for not shipping them via the USPS.

Five more eBay lots are ending tonight. Tomorrow, you can bet that I’ll be taking my packages to the post office, even if it means waiting it long lines and dealing with clerks who seem to be mumbling, “I wish you all would just go away” under their breath. Despite the hassle I am not going to make that same mistake, of losing money on shipping, yet again.