使用键/值表优于可空列或单独的表的[dis]优点是什么?

I'm upgrading a payment management system I created a while ago. It currently has one table for each payment type it can accept. It is limited to only being able to pay for one thing, which this upgrade is to alleviate. I've been asking for suggestions as to how I should design it, and I have these basic ideas to work from:I'm upgrading a payment management system I cre