Signed-For service instead of a postage discount?

I ordered two items on eBay from the one seller and I assumed I would be able to negotiate a combined postage discount before finalising payment.

However, I was instead charged immediately for two separate postages​.

I emailed the seller and asked if he could combine both items and if I could get a postage discount.

He replied with the following;

"Hello,

We upgrade orders with multiple items to the Signed For service instead of offering a postage discount."

I'm not well up on what a Signed-For service is. And is it a good thing?

Any help would be appreciated.

Thanks.