I don't think that would be a problem usually. Most purses weigh between 2 pounds and 5 pounds, even with packing materials around them. A lot of mine are under 3 pounds so I buy the $8 label. If I'm worried that it might weigh more, I buy the $13 label. Nothing ever goes over 10 pounds unless you are selling something heavy like garden tools or a cast iron pan. If you pack it up and realize you specified the wrong label, you can change it then and Mercari will adjust the amount you earn.
If you buy your own postage, besides charging by weight, USPS also sometimes adds a surcharge for oversized boxes, which could be anything over 12 inches on a side. It depends on the overall size. Like the boxes
Coach uses to ship purses in are considered oversized. Depending on the zone of the recipient, this surcharge could be huge. I've calculated some boxes that were 3 or 4 pounds, and it would cost $40 to $60 for me to ship them. You can use these Mercari labels even on these oversized boxes and not pay extra.
It's worse for me because I live in California. So some packages have to go all the way to the other side of the country. A package to New York, even if it isn't oversized, is still around $15 for 3 pounds. Mercari labels are best for me.
I usually just estimate the weight when I set up the listing. I have a postal scale that I use when I actually pack it up. A scale doesn't cost much and is worth it if you are going to do a lot of selling.
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I've loved this thread so much. Your support, kind words, generosity of spirit and gracious humour have made visiting with you a delight and something to look forward to every day. I've loved talking about handbags and everything else in between. 
