Extra weight being added to all items

I've got a weird one. I've been going through the shipping options after installing the UPS and USPS interfaces. I already have accounts with both services for my existing online store, and had no problem setting them up in ZenCart.

When you go to 'estimate shipping' the line at the top shows the correct weight for the item in the shopping cart. But all the shipping options show three pounds extra, so of course, all the prices are wrong. This is true for every shipper and every product, so it's not just an anomaly in how I set up one particular product.

Here's an example. Anyone have any ideas?

Never mind, I see someone else has already asked about this, and found the problem: set tare in admin shipping options.