You need to purchase the barcodes in blocks, or else will likely clash with someone else's barcode if you use them for retail or anywhere outside your business.
The barcode people get very protective if they become aware of any abuse of their barcodes, which are regulated.
Bartender will output barcodes from numbers input in the correct format, and generate the last digit automatically, which is a check digit. There is a free trial of Bartender available on the Seagull website, but when the trial ends, the software inserts incorrect characters in the barcode.