@Stackss
I would kindly suggest you use any free time you have, not easy I know what with the intensive supervision of aggressive revision regime in place for 'the twins', to teach your husband a few tricks for his new job.
You could teach him to hover after presenting the Chow Mein to elicit as large a tip as possible. He should blow on his hands whilst hovering to indicate just how cold it is in order to garner (greater financial) sympathy from the recipient.
When delivering to schoolmates of 'the twins' he should be very friendly (without being pervy, of course) and entertain them with his 'Dad jokes' and remind them 'the twins' love that sort of humour.
When delivering to the parents of the schoolmates of 'the twins' he should avoid all eye contact and pretend he's his own brother/cousin if actively challenged. The parents will be obviously and extremely awkward when faced with him at their front door. The fact he's clutching one jalfrezi, one dhansak, one bhuna (to be shared by their own twins) and assorted accompaniments will increase that bewilderment.
Naturally, it's likely the other twins' parents will be very confused around tipping etiquette to the man in the big house, in a naice area, in his middle class job putting his twins through private school.
Their minds will also be whirling and wondering how quickly they can draft a wee newsletter to all the parents of their twins' schoolmates to share this nugget before your hubby delivers the KFC to the Smythe-Winterbournes and they grab the scoop.
It's important, when hubby is aware of this, and he has not been directly challenged, he employs a false accent, perhaps Italian?, to further befuddle the recipient and maximise the gratuties whilst he can ......
Oh, and have him report to you after each delivery he makes confirming the fee plus any gratuities. This is crucial. He needs to be aware of who is in control.
HTH.