'Cash purchases' means any payment you make to suppliers with whom you do not have an account. As you don't have an account, you will have to pay for the goods at the time when you buy them.
You might make Cash purchases from a DIY or hardware shop that you use for one-off or infrequent purchases of general items like disposable gloves or hand cleaner.
Cash purchases only include items that you buy for use when working on a vehicle. They don't include things like tea and coffee bought for consumption at work, or items such as business stationery.
Remember to keep safely all the invoices and receipts for Cash purchases, even if the amounts are small.