USPS can, technically, handle parcels up to 70 pounds. But UPS and FedEx are way cheaper than USPS for large heavy things (for businesses, not for individuals!) so most companies won't send it USPS.The only problem I have with buying anything online is that UPS claims my address doesn't exist and USPS will not do anything over 20 lbs.
If there's a FedEx store in the town you shop in, they let you pick it up there (Chewy uses FedEx, at least for me).