Why 4×6?
Many thermal shipping-label printers expect a true 4-by-6-inch page. A marketplace PDF may instead place that label inside Letter or A4 paper.
Crop before scaling
Choose the part of the source page that actually contains the shipping label, then Quicklio fits that region proportionally onto the 4×6 output.
No raster screenshot
The PDF page region is embedded directly into a new PDF, so text and barcode/vector content can remain sharp.