I found this in  your handbook and found it very helpful and great.
Is there a way for me to influence the calculation, so that it doesn't add an extra $20. 
Quoted from the FAQ:
[when using the "prepaid balances" built-in Foodclub accounting system, show an Amount due for each user, next to their Prepaid balance on their nav bar and also on the User maintenance page for admins; amount is calculated as: [cart total] - [prepaid balance] + 20.00  (asks each user to pay 20.00 extra, to keep a buffer for price changes)
[#larchives li#amountDueDisplay,
table#adminUserMaintenanceTable td.ActionCol div.amountDueDiv {
  display: inline;
}]
Thanks for any suggestions,
Suzanne
			
		