NOTE: You may want to consider creating one letter for back office payments and another letter for sales order payments. The steps below walk you through inserting a merge field from the order record, so this does not apply to back office payments 
  1. In Marketing and Communications, click Acknowledgements
  2. Highlight the letter, click Edit
  3. Click Edit in the you have defined you mail content section
  4. Click Edit again to open the letter
  6. In the Merge Fields drop down, click More Fields...
  7. Expand Revenue Marketing, Revenue, Sales Order, Sales Order Payment
  8. Highlight Revenue, drag Receipt Amount to the selected Fields
  9. Click Save
  10. Select and insert this field from the merge fields drop down list
  11. Click Save