Acceptable time for payment? How to handle "friends" as clients

I figure this is a good place for this here. I'm just looking for an opinion.

I'm a successful self employed individual in the tech space and I have several revolving 5 figure monthly contracts with people / businesses I know and trust that I've been doing business with for 5 - 10 years or more.

payment is sent on time every time on a monthly basis.

I have one "client" lets call a somewhat close friend who I occasionally do side work for on weekends on a quarterly basis. Its a simple task that I've spent the time and effort to "tool" out I know I know he can not get it done cheaper elsewhere and within the timeframe he often "demands".

The issue with this "friend" is he knows part of my business and the success that comes with it. Often I have to bug this guy for payment somewhere between $3k - $5k quarterly billing. But I've always said to him since his product tends to be time sensitive that I expect payment upon delivery. There have been multiple occasions where payment either isn't made upon delivery or when he says he's going to make it, he doesn't.

I've never not been paid but just this past month I made another order up, he said he would pay full upon delivery and then it turns out on the spot he had "half" but no issue he'd pay the rest within a week. It didn't happen so I messaged him once per day asking for the payment transfer.

3 weeks later it was made. I sent one message per day. When the payment was made I was told "I needed to calm down about it and that I should know it would be made" Like honestly how do I even handle this?