First, keep the shipping refund.
Second, you should never use a "guesstimate" - when you create your listings and when you buy your shipping labels, always use the actual weight and outer dimensions of the package. Know how you're going to ship before you list.
Third, the post office just scans the label and then the label is used. It's done. They will not tell you if it's accurate or not as it's a prepaid label. Unless you're doing something grossly negligent like sticking a Ground Adv label on a Priority branded box (not allowed), the clerk will accept your package, scan the label, and send it on its way. Any cost adjustments for under/over payment are to be handled by USPS APV process, an automated system that audits size/weight of packages in transit.
If you don't have a postage scale, buy one. They are inexpensive and it's needed if you plan to keep selling and shipping.
