Adding USPS Flat Rate Boxes

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I would like to add a USPS Small Flat Rate Box to my Shopify shipping options (by setting it as a customized flat shipping rate). If I understand correctly, as long as the price set with this customized flat rate box is the amount that will cover the discounted Shopify flat rate amount, it will be shipped with no issue, right? 

In othe words, I'm just wondering how USPS knows that the shipping label is coming from Shopify and will accept the discounted price label on the USPS flat rate box (normal USPS flat rate boxes are $8.30 to mail, but the Shopify price is cheaper)?

If anyone has experience in setting up flat rate boxes in their shipping section I'd love to hear how you did it, and any tips.

Thank you!