hatwoman - you're making a very common mistake here, which is to make it personal. Unless there is someone at your old firm who is intentionally not paying you, then you're not dealing with people, you're dealing with a system. As it's a huge corporation they will have an accounts system that, once it has been started, can't be stopped. The key is to make sure that your invoice gets into this system.
You have an advantage here, though, in that as you worked for the organisation, you should know how the system works.
The person who you invoiced will be responsible for signing off your invoice and then passing on to the accounts department for payment.
TBH, your 30 days are unlikely to be taken much notice of as the company will have their own time frames that they operate within. Many organisations pay at the end of the month after the month in which your invoice was received - so you could have to wait up to 60 days in some instances.
The critical things to do are:
- make sure that the invoice has been signed off/aproved
- make sure that the accounts department has received it, and that is has been entered on to the system. Unless this has happened you will not be getting paid!
Also be aware that they will have to set you up as a new supplier on the system, and this in itself can take time, which would also delay your payment.
The best thing that you can do is to make a friend of the person in accounts payable, to find out the status of your invoice. When you have this information you will be in a much better position to decide what to do next.
As well as finding out the status of your invoice, make sure that you also find out (if you don't already know) how their system works - i.e. once they have received an invoice how long does it take to process through to payment. Then you will know how best to invoice your next job.
Don't turn down the chance of more work - the person commissioning you almost certainly will have nothing to do with the 'late' payment (unless they have forgotten to pass your invoice on/deliberately not passed your invoice on). You will undermine your chances of further work if you do this. Unless this person can specifically influence payment of your invoices once in the system (which is very unlikely), you need to leave the issue of payment alone and deal only with the issue of more work.
And when you do the next piece of work, the best advice I can give you is to make sure that you phone the accounts department about a week after you have submitted your invoice to establish whether or not it is in the system. If not, then you can contact whoever is responsible for signing it off and ask them to make sure it's been passed on.
Sorry this is so long, but it is based on 10 years' worth of dealing with different companies and their assorted payment systems!
And good luck with future work/payments.