When you charge customers based on what a scale reads, that scale is a commercial weighing device, and weights and measures law applies. In practice this means using an NTEP-certified scale (look for the NTEP Certificate of Conformance number on the data plate or in the manual) that meets the accuracy class appropriate for shipping, typically Class III. Consumer kitchen scales and uncertified "shipping scales" from online marketplaces generally don't qualify, no matter how accurate they seem.
Most states also require commercial scales to be registered with the state weights and measures office and inspected periodically, and some require a registered service technician to place a new scale into service before first use. Requirements and fees vary by state, so check with your state's weights and measures program. Inspectors do visit retail shipping stores, and an uncertified scale is an easy citation.
Any NTEP scale that PostalPoint can read over the industry-standard USB HID POS protocol will work. If you're buying new, confirm the capacity covers the heaviest parcels you accept and the division size is fine enough for letters and small packages.