I think Melio does that so the can charge $1.50 per check.  I agree, I'd like to pay multiple invoices with one check, it's more convenient for my vendor as well.

I'll add that Melio seems it's their right to contact my vendors to ask them how they prefer to get paid.  If they answer that they're OK with a one time use debit card, they'll make that the default method and you can't change it.  This is another Melio scam forcing your vendor to pay a 3% processing fee.  They need to stay out of my business...