Frank,
You just weigh the package, and enter the weight in the spavce provided. I picked up a scale on eBay several years ago for $20.00 dlvd. It's a weightMax digital postal scale. I don't recall who I got it from, but it has performed faithfully on with only one battery (9 volt) change in all this time.
I use DC for all my packages. I use eBays shipping labels, and have no worrys at all. I also do not offer a refund policy. In my store policies, I let buyers know I stand behind my items, but I also let everyone know I don't intend to waste postage and time on buyers remorse. Also, if you are concerned about shipping fees, use the shipping calculator. State in the item description that you will add, say .75 for handling (including DC and bubble mailer). That way, buyers know what they are getting for their charges. With up to 3 ounces being the same price for 1st class, free combined shipping should be a no brainer, and should increase sales a little.
Right now, I'm not selling cards. But I believe I have my store set up in a way that makes both buying and selling easy. Check it out.
---Frank