I don't think anybody really associates couriers with sellers - they're part of the infrastructure. It doesn't really count as "contact with the customer" (unless you see it as a business opportunity - making the experience special - which for some reason no one takes).

E-commerce businesses do however have strong incentives to ditch the completely unreliable delivery companies, because they cause a lot of mess on the business end (besides angry customers, you waste a lot of time finding the package and scolding the delivery people). As a result, most delivery options suck equally.