First, please know that Return Magic does not bill you for or add any fees to your return labels. Our charges are associated to the use of the customer portal & dashboard to process returns and gather data on your returns, so if you do not want to offer return labels to your customers, you do not have to.
With Shippo, when you are billed will depend on the type of labels you are generating. If you are using one of USPS, UPS, FedEx or Australia Post, and you are using the same carrier to ship out your orders, then you are generating scan-based return labels. Those labels allow you to only be billed when your customer actually uses the label (i.e. it is scanned by the carrier).
For any other carrier or any other case (e.g. you are using USPS for returns but not to ship out your orders), then you are billed for the label when it is created. This is when your customers place a return request and their return is approved.
Shippo will bill you when you've exceeded $100 in purchases, or after 7 days have passed since your last invoice (whichever comes first). There's more information on this here.
For their different pricing options, please refer to goshippo.com/pricing.