Checkout system with offline payments

I'm trying to find the best way to handle offline payments (e.g. internet bank transfer) into a site which offers services like courses.

At present there is a course registration form, when successfully completed the user receives a confirmation screen and email which states that payment is required, confirms the amount and bank account details.

I'm concerned that when a user completes this process it may feel as though they have achieved a booking or reservation, regardless of the information that follows.

For management a difficulty is that once the checkout is completed it takes a minimum of 24-48 hours before the payment can be confirmed. Also, the user may choose to not pay immediately (or at all). During this time the list contains unpaid booking requests, and it's proving hard to manage the attendee list and be sure of who is coming.

I'm wondering if anyone has encountered this problem before and if there is a better way to handle the checkout process.

More information:

The UX objective is to have an easy to use system for a user to create a course booking, where if they choose to pay 'offline' it is clearly understood that there is no reservation of a place until the payment has been made.

It could simply be that a standard process like the current one is fine, we just need to work the copy so that it communicates this well.

Another possibility I've considered is a system where 'offline' payments are completed before the booking form is completed, identified using a unique code sent via email. This might better reflect reality but it would also be pretty unusual and could put people off.