On the day of commissioning we have the prepared handover book, 2 copies, we complete the electrical testing certificate and duplicate that to the customer's copy. We present the book tot he customer, explain it's contents ask for a signature in their copy and ours and give them an invoice with bank details.
We ask for a bank transfer that day explaining that if they don't do online or telephone banking that they can show the invoice to the counter staff who'll make payment without charge.
Often we hand over the invoice at the beginning of the last day so that they can make payment while we commission.
Once we confirm the payment is received, normally that evening we produce the paid invoice and MCS cert and either post or email it to the customer. If the customer seems a bit baffled by it then I take it round and complete their FIT application form for them too.
We do still get handed cheques and I don't refuse but I do ask again if they can do bank transfer. About 1 in 8 pays by cheque.