Always have clients pay ad platform costs directly with their own payment method. Paying direct ad costs yourself and then billing them is a huge liability, plus if they pay you by credit card you will need to add a processing fee; this adds no value for anybody.
As for billing them for your services, just bill them in advance at the beginning of each month. We’ve been doing this for 5-years without any issues.
If a client has decided to trust you to manage their money in terms of advertising, the payment terms aren’t much of a sticking point. In any event, they can always do a credit card chargeback if you don’t deliver what’s agreed.
I’d say we only get about 1/50 prospects that challenge our terms. If somebody isn’t willing to work with you on this basis it’s probably because they have cashflow problems or aren’t trustworthy themselves.