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).
I'm not an expert on this, but a friend who is an Amazon seller recommended getting a GS1 tag and going from there. The website provides clear and easy-to-follow instructions.
If you have questions on building your organization, product and business strategies, shoot me a message!