Shipping to Amazon FBA — Is it sometimes free?

Hi All,

I’ve been selling on Amazon for a week now.

Earlier in the week, I made 4 shipments to amazon. Each shipment cost me about $4 through UPS.

Then, yesterday, I decided to make 2 more shipments to amazon, again through UPS. However, amazon didn’t charge me for the shipping label (it was free).

Can anyone explain why I didn’t have to pay anything to ship these 2 last shipments to amazon?

Thank you!

Best,

Yippiekayaye