Can I create an invoice with a predetermined amount of payment. Example customer pays me ksh 1500 irrespective of what I incur to give them services and drugs

Let's make sure you'll be able to record correctly the amount that's paid, Priscilla Mwendwa.

I have another way on how you can record this in QuickBooks Online (QBO). You can create an invoice with the predetermined amount. Then, after the service incurs, you can enter a bank deposit wherein you can match the payment you received from your customer. Let's say, you created an invoice with the amount of 1500, but the actual cost is only 1300. So you use the actual amount that is the 1300 when receiving the payment.  To do this, follow the steps below:

1. Click the + New icon.
2. Select Receive payment.
3. Enter the customer name, payment method and amount received 1,300.
4. Click Save and close.

Then, create a deposit. Here's how:

1. Select + New.
2. Select Bank Deposit.
3. Under the Add funds to this deposit section, enter the remaining amount (200) to match the invoice.
   
   
4. Click Save and close. 

That should do the trick. In addition, QuickBooks automatically downloads the latest transactions. To learn more on how to categorise them, read this article for more details: Categorise and Match Online Bank Transactions In QuickBooks Online.

Feel free to let us know if you need further assistance. We'll always be right here to help you anytime.