Another option is to use the point system where you set up the customer on the system and for instance they pay in advance you can say give them 10% extra on their payment so if they paid £50 + 10% they get 55 points (£55) as each point is = to £1. By serving them up this way you can email receipts and if they spend £10 but only have 5 points (£5) left the till will be charge them the additional £5 when charged. They can also prepay at any stage to add to their point system. We used this system and it is so easy to use and as there is no cash or card payments on each purchase other than initial payment it saves time for staff.
If it’s a composite item and you have set the measures of each ingredient for the item then if you put it down to Ullage with 100% discount it would take the measured items from your stock levels. You can then go in to discounts and select discounts, set the period day/week/month/year and it will give you a ullage figure at resale price. This info would be enough for your accountants as you have a profit margin in your item list for each item so can work out the actual loss at cost price! We use it in a bar but I can’t see if it would make any difference to what ever you sell as long as the measurements of each ingredient is calculated then linked with composite option.