First, avoid UPC codes.

They're expensive + cumbersome + Amazon no longer uses them... or rather... even if you have a UPC code, you'll require replacing these with the Amazon AISN number assigned you.

https://www.gs1us.org - UPC Mafia, where you purchase UPC codes.

More important is a high throughput multicolor printer (to print ad-hoc labels) + a barcode reader device, to ensure your barcodes are accurate (can be read by Amazon barcode scanners).