The best answer is, you don't need to. Allowing selection for specific carrier services or methods is simply not in a seller's best interest, especially when there are outages at UPS or FedEx.
Instead, we recommend only offering free, tiered, or flat-rate shipping at checkout. This makes fulfilling orders less rigid and increases sales by reducing the amount of indecision possible during checkout.
With that said, SKULabs does not directly provide rates to your Ecommerce software, however most software today already supports for calculated shipping rates during checkout.