My brother has a nice, rigid tonneau cover on his Ram 2500. But when he had to move a sofa for my mother - about 1.5 hour drive - it was beginning to rain. The tonneau had to be removed to fit the sofa - so we put on a flexible Bedder Cover. We even had to leave the tailgate down. No problem! We just kept wrapping Bedder Cover until it was all sealed up. Just like plastic wrap on a plate of food!
Tonneau covers are great for a more permanent truck bed cover but they are expensive (soft ones are about $700 and rigid ones over $1,000), take a good bit of effort to install, and can't accommodate large loads. Bedder Covers don't replace tonneau covers - they are a one time use item.
But, if you have a big sofa you want to transport and keep dry you can't use a typical tonneau cover.
Try a one-time use Bedder Cover!
A couple months ago I was helping a friend move some boxes - another friend had a pickup with a three-fold, hard tonneau. The boxes were too big for the tonneau, so we had to move it out of the way.