I'm Darren, one of the Gurus here at Shopify and I'm glad to be at your service!
Using Stamps.com is a great idea for using as your shipping tool, we have a great app available from Stamps.com that allows for integration with your Shopify store! It fully integrates with your store and there is also a great 4 week trial with special offers on digital scales and label printers for Shopify subscibers!
If you have any trouble with that or if there's anything that you're uncertain about, don't hesitate to contact us again at any time. We're open 24/7!:)
Customer Success Guru