Nobody reputable offers 5% interest net of tax (but do post the details if I'm wrong) and, even if you could, that would be for a year !
Most people would want to have their course or hours in a month or two, so any interest would be almost nothing.
A cheap package price or a large discount off pay-as-you-go prices is a good way of saving a lot of money BUT make sure you are using a school which has been in the same ownership for long time. Also, make sure you are paying the course provider or hirer not an agent or the instructor.