Each shipping carrier has different policies, resources, and requirements. Make sure that you understand the specifics of the shipping carriers that you purchase labels from to avoid any shipping delays or unnecessary fees.
Carrier-calculated shipping rates through Shopify Shipping often show expected shipping times next to the shipping rate, however, they don't take into account your processing times. For more information on processing times, refer to Understanding and reducing processing time. For more information on letting your customers know about your processing times, refer to Adressing processing times.