Two pages, two origins
US Letter is 8.5″ × 11″ (215.9 × 279.4 mm). A4 is 210 × 297 mm. Letter is wider and shorter. A4 is narrower and taller. If you send a Letter PDF to a driver set to A4, the driver typically shrinks or letterboxes. Either way, a 30-up grid that was built for 0.1875″ side margins is no longer on those margins.
LabelStack presets set the PDF page box to the paper the sticker sheet actually is. Avery 5160 / 5163 / 5167 and the US FBA sheets are Letter. L7160, L7163, and L7651 are A4. If you are holding a sheet from a EU office shop, look at the millimetre page size on the wrapper before you pick 5160 because “it looks like 30 labels.”
L7160 is not 5160 in millimetres
Avery L7160 is a common 21-up A4 address sheet: 63.5 × 38.1 mm labels, three columns, seven rows. That is not a 5160. The label is larger, the count is 21 not 30, and the margins are metric. Using the 5160 preset on L7160 stock will print 30 US-sized rectangles onto a 21-up European liner. Using L7160 on 5160 stock is the opposite mess.
L7163 is a larger A4 label (often used like a shipping sticker). L7651 is a small 65-up. Read the product code on the box. If the code is 5160, 8160, or 18160, you want the 5160 preset and Letter paper in the print dialog. If the code starts with L7, you want the matching L7 preset and A4 paper.
Print dialog: paper size is not a suggestion
Chrome will happily show “A4” as the paper even when the PDF is Letter, because the last job was A4. Set it every time. Scale 100%. Actual size. The alignment guide covers the rest of the dialog; this page is the paper-size half of that failure.
If you only have A4 in the tray and the stock is Letter 5160, stop. The sheet will either jam or print off-grid. Buy the paper that matches the liner, or buy liner that matches the paper. There is no software offset that turns Letter into A4 without moving every sticker.
How to measure a mystery pack
Count the labels. Measure one label’s width and height. Measure the left margin and the top margin. Measure the gap between columns. Compare to the preset list in the how-to page. If none match, Custom size: type page width and height, label width and height, columns, rows, and margins. Export one PDF page and hold it against the liner at a window. That five-minute ritual beats a 50-sheet reprint.
Sellers who split inventory between a US 3PL and an EU 3PL should keep two working CSVs or at least two export runs: same values, different templates. The barcode encoding does not change. The page box does.