Put an item on the book by hand. Most items come in from invoice photos; this is for anything that didn't.
Already on the book? Pick it instead:
Load the register's price book (on Passport: Setup › Store › Third Party Data Interface › Export Pricebook) to price everything at once, including items that haven't sold yet. A price from a register sale is newer information and wins over the price book whenever the sale is more recent.
Load the register's sales journal files (PJR….xml from the Passport back office). Each sale comes off stock once; loading a file again never counts it twice. A sale whose barcode isn't on the book waits below until you say which item it is.
Pick a CSV exported from your current back-office software. The header needs a barcode column and a name column — barcode, upc, sku, item_code and plu all work, as do name, description and item_description. Cost, retail, category, sub-category and pack size come along if they're there. A category can also be written as one column, like Tobacco > Cigarettes.
Photograph the invoice or packing slip flat, with the whole page in the frame. Each line is checked against the invoice's own math and totals. Nothing goes into stock until you press Receive.
A number of your own, worked out from the items on the book with the same costs and prices as everywhere else.
Every movement is logged with a timestamp. These counts are what was received, taken out, and sold at the register in the sales journal files loaded under Register sales.