Overhead absorption for service company

Londina

I've been look for Overhead absorption calculations online but what I find is mainly about manufacturing business and labour hours.

What about if a service business wants to include fixed overheads such as wages, rent, bills, expenses in their profit margin calculation so they know that their selling price covers indirect costs too, not just direct costs?

Example: an online shop place an order for £8000 worth of items with a mark up 60% which give me a sales figure for £12800. Their monthly overheads are around £5000, how I apportion this to this order?

Thanks in advance