ESA card the same as a letter from a medical professional?

[Ramsey Hantash]

Prospective tenant has an emotional support animal and sent me one of those cards that you purchase online (assuming). I would like to ask for the official letter from a medical professional, but wanting to tread lightly on this. Do those cards online suffice? Or am I allowed to ask for the actual letter? The card says "This animal was prescribed by a physician or mental health provider"

Context: i'm ok with the dogs regardless but I want them to pay a security deposit and they are saying they don't need to pay a deposit with the emotional support animal.