There really aren't many options out there. Amazon has a strong monopoly. You might want to check out google books or an eReader specific service.
There's Hoopla Digital, which is a free comics/eBooks/Audiobook/Movie service supported by libraries. If you have a library card you can check to see if it grants you membership. It's well supported by most comics publishers, with the exception of Marvel. Marvel is definitely on there, but it's selection is almost nothing compared to the many others that offer their collected editions day of release. If you have enough borrows in the month for what you want anyway.