How to create a subtotal for certain products

Hi, I have a booking form with multiple products and I would like to create various sub-totals that only add up some of these products at a time, eg camping, equipment, site and conference fees. Can you tell me how I would do that?

The form can be seen at http://www.newlifechurches.co.uk/booking/. There is no add-on being used and no payment being taken. It is just to allow people to submit a booking form and I've kept it simple by using a product for each booking option.

I've seen calculation fields mentioned in the forums but can't find documentation on how to use them.

Thanks,
Paul

Thanks but those release notes don't say whether it is possible to create a subtotal of products using the calculation field. For example how would I use the number or product field to add up the total from a selection of other product fields used in the form?

eg Product A £10, quantity selected 1
Product B £15, quantity selected 2

I now need a subtotal which shows the total cost of product A and product B including quantity which should show £40.