There is no single rate model that fits every subscription business. The right choice depends on how consistent the box is, how widely customers are distributed, and whether shipping is shown as a separate charge.
Real-time carrier rates are useful when parcel characteristics or destinations vary materially. Calcurates can connect carrier accounts and request checkout prices based on order weight, dimensions, destination, service, and the applicable package. This keeps the customer-facing amount closer to the carrier calculation.
Custom Shipping Options are useful when the business wants a simpler promise: one flat charge, free delivery above a threshold, or table rates by destination and order conditions. A custom rate should still be tested against actual costs. Simplicity at checkout does not remove the need to monitor margin.
A practical ecommerce shipping software setup may combine both approaches. For example, a store can offer a predictable economy option based on a custom calculation and an expedited option based on a live carrier quote. Multi carrier shipping software also makes it possible to compare available services instead of assuming one carrier is economical for every zone.
Whatever model is used, shipping rate calculation software should receive accurate package data. A live rate based on the wrong box is not more reliable than a carefully maintained flat rate.