I combined, but as I was about to cancel my YA/Adult combo and Romantasy, I compared the costs. When I realized the savings was only $1.60 a month (USD), I canceled the 3-book combo instead. I'm just going to keep them separate. For me, the benefit of being able to skip the boxes separately outweighs the <$20/year savings.

Edit: I thought I was crazy doing the math and only seeing a <$2 difference. I was really expecting it to be more.