Most marketplaces hand you a shipping label on a letter-size page with three quarters of it empty. Thermal printers want a 4×6 label and nothing else. LabelConvert closes that gap in about two seconds, without asking you to upload anything.
The page is drawn at high resolution inside your browser, then scanned for ink. Bands of pure white split the page into separate blocks, and each block is checked for a tracking number or a dense barcode so a packing slip can be told apart from a label. The label block is then cut out of the original PDF as vector art, not as a picture, so barcodes stay razor sharp on a 203 dpi printer. If the automatic box is off, you can drag it yourself.
No uploads, no accounts, no storing of label data, no selling of anything. The full detail is on the privacy page.
Because everything runs on your device, running this site costs almost nothing. There is no paywall and no label limit. You can install it to your phone's home screen and keep converting with no signal at all.