When entering mastercard payment as expense, should I enter the total amount paid to mastercard as 1 expense ("Mastercard payment"), OR should I list each item separately

Ordinarily you would want to use a Credit Card account to track and categorize your MasterCard purchases individually as you make them. This also lets you reconcile your records against the bill when it comes, and you can spot billing errors and other issues this way. When it's time to pay the bill, QuickBooks will record the payment as a transfer from your checking account to your MasterCard account (maybe by way of your Accounts Payable account).

If you record your payment as a single expense, you're losing potentially important information. For example, if you're buying items for resale, items for business use, and paying individuals for goods and services, each of these gets different tax treatment that will cause tax reporting problems if you're not tracking them properly.

You really should consider using a Credit Card account to record and reconcile your expenses instead of just entering a payment to your MasterCard account.

The payment against a credit card is never an expense. It is only a reduction of loan liability from incurring expenses as credit card charges. If you enter the expenses in your check to the card company you are not going to be correct as far as when the expenses occurred.

Proper work flow is to enter the charges, one by one, as Expense.  This posts to the cc account that you set up if you do not already have it. Credit Card type = and you will be able to reconcile it to the monthly statement. Write the check before or after reconciliation but the check from banking pays the cc account and is not an expense itself

Nice to have you in the Community, @nancy12. I'd be glad to help with entering the Mastercard payment.

Generally, you would want to select just the credit card account to record the payment for. Should you be referring to listing each item purchased and assigning it an expense category, it will be duplicated should they already be recorded. 

There are a few different ways for recording a credit card payment in QuickBooks Online. I recommend reviewing the following article for further insight into this: How to Record Credit Card Payments.

I believe that this will help to get you on the right path. 

Please let me know if I can be of additional assistance. Wishing you well.