What is the cheapest way to send parcels as a business?

If your in the UK, check out parcel to go.

The cheapest is not always the best and if your customers are not happy with the service your business will not last long. My hermes is cheap, sometimes the royal mail/parceforce is cheaper though.(my hermes has a limit so only ok for small businesses really)

It really depends on what you are sending, what size, which country , where to etc

It depends on where you are, where you are sending the parcels, and how big/heavy the parcels are. Without that detail there is no right answer.

As Richard said, too many factors. In one instance it might be UPS, another FedEx, another USPS.

Give your customers options on the carrier and have the total calculate for them and let them pick.