Long time no see, Conceit!
The advice given that you should not work on an hourly basis is bollocks. Yes, usually with big companies you have no choice, but sometimes start-ups and individuals prefer to work on an hourly basis. Sometimes I prefer working on an hourly basis, sometimes I prefer working on a fixed price, it depends on my availability at the time, how experienced I am with the type of project (and thus how well I can give a quote). You may want to do hourly first then switch to a fixed payment.
Regarding your original question,I've only had to send an invoice 2 times within 6 or so years out of all the jobs, and I've managed to get my payment without an invoice... but the only thing I needed was my ABN number... In Australia I had to register a company in my name, and provide my business number. Probably depends what country you're in.
Regarding the day job/overtime remark, many people don't have a choice about whether they get paid hourly or paid a salary at their day job, unless they quit, but how realistic is that?