Since this is a non-inventory item for which you don't maintain a physical count, you may bill the customer for this "noninventory" item on an "A/R Voucher" as opposed to a "SALES INVOICE" (and running the transaction through inventory.)
 
To create the AR Voucher, go to DBA Sales module, Accounts Receivable, select B, enter the customer code, create an invoice number, type of voucher is AR voucher, enter date, customer terms, brief description, dollar amount of sale, for G/L account, select the appropriate sales G/L  - perhaps Misc. income or your normal Sales revenue account.
This voucher will post directly to Accounts Receivable for the customer.  There is no need to "post" this voucher as one does for a sales invoice.
 
Since you don't maintain an physical count of this item, why would you want to create an adjustment to inventory for it for this customer sale?
 
I think the AR Voucher method will be a lot quicker and easier.

An AR Voucher is an "invoice".  Simply print it through SO_F and enter the voucher number you had assigned.  (to the question, do you want to print an unposted invoice - answer "no") and at the bottom of the screen where it says "invoice type to print" - select "AR invoice"
 

However, AR Voucher is still a useful tool for billing for non-inventory items and for those items which do not have a COST of GOODS SOLD (i.e. services provided, expediting  fees, etc.)