What is actually happening
eBay hands you a PDF whose page is 4 inches wide and 6 inches tall. A #10 envelope is 9.5 inches wide and 4.125 inches tall. Those are two different pieces of paper, and when you send the first to a printer loaded with the second, the driver has to decide where on the envelope the smaller page should land.
Almost every printer driver answers that question the same way: center it. That is a reasonable default for a photo and a terrible one for postage. So the label ends up centered on the envelope, and because the label page is taller than it is wide while the envelope is wider than it is tall, it is usually rotated wrong as well.
Why the position matters at all
It is tempting to think a centered label is cosmetic. It is not. USPS sorts letter mail on automated equipment that expects to find things in specific places: the postage indicium in the upper right, the return address in the upper left, and the delivery address and barcode in a band across the middle of the envelope face.
A label dropped in the center of the envelope puts the postage roughly where the delivery address belongs and leaves the upper right empty. At best a machine rejects the piece and a human sorts it by hand, which is slow. At worst it is treated as unpaid.
Three ways to fix it
1. Give the printer a page that is already envelope-sized
If the PDF page is 9.5 by 4.125 inches, there is nothing for the driver to center. The page and the envelope are the same shape, so everything lands exactly where you put it. This is the fix that survives changing printers, because it does not depend on any driver setting.
That is what the eBay Standard Envelope Converter does. It cuts the return address, postage, and recipient block out of your label and rebuilds them on a true #10 page, at their original size.
2. Nudge the label with driver offsets
Some drivers expose a manual X and Y offset, and you can push the centered label toward the corner. This works, but it is worth knowing what you are signing up for: the offset lives in the driver, not the file, so it applies to everything you print until you undo it, and the numbers are different on every printer model. Plan on several test envelopes to dial it in.
3. Print on adhesive label stock instead
Print the 4 by 6 label onto 4 by 6 adhesive stock, peel it, and stick it on the envelope. The position problem disappears because you are placing it by hand. The tradeoff is that you are now buying label stock for something that could have been printed directly on the envelope, and a 4 by 6 label on a #10 envelope overhangs the edges.
Do not reach for Fit to Page
The instinct when a print lands wrong is to turn on Fit to Page, Scale to Fit, or Shrink Oversized Pages. Resist it. Those options change the physical size of everything on the label, including the routing barcode and the postage indicium.
Barcodes encode information in the width of their bars. Scale one to 94 percent and the bars are no longer the widths the scanner expects. Sometimes it still reads. Sometimes it does not, and you find out after the envelope is already in a mailbox. Leave scaling at Actual Size or 100 percent and solve the position problem by changing the page, not the size.
Check it without wasting envelopes
- 1Print the converted PDF onto a plain sheet of letter paper first, at Actual Size.
- 2Lay a real envelope on top of the printed sheet and hold both up to a window or a lamp.
- 3Look for the postage inside the upper right corner, the return address inside the upper left, and the recipient block clear of all four edges.
- 4Measure the routing barcode with a ruler and compare it against the same barcode in the original eBay PDF. They should be identical.
- 5Only once that lines up, load an actual envelope and print.
If the alignment is close but consistently shifted in one direction, that is a feed problem rather than a page size problem. Check how the envelope sits against the guides in the tray, and confirm you picked the right feed orientation in the driver.
If it is still landing wrong
Work through these in order, because each rules out the one below it.
- Confirm the PDF page size really is 9.5 by 4.125 inches. In Adobe Reader it is under File then Properties.
- Confirm the print dialog scale says Actual Size or 100 percent, not Fit or Auto.
- Confirm the driver paper size says Envelope Com 10 rather than Letter or A4.
- Confirm the feed orientation in the driver matches how the envelope physically sits in the tray.
- Turn off borderless or edge-to-edge printing, which can shift the whole image inward.
The print settings guide walks through each of those for Chrome, Adobe Reader, and macOS Preview specifically.