I Have A Really Important Question Regarding Paypal. Help?

If they try to chargeback then they have to go through the bank so paypal will freeze just that transaction.
You get contacted and get 10 calendar days to reply with any information they have requested to see if you qualify for paypals seller protection policy.
Ultimately its outside of paypal whether the chargeback is valid and the final decision is up to the card company.
If your covered by the seller protection then paypal will un- freeze the transaction and return the money to you.
If your not then the transaction will be reversed in your account.
If Paypal have enough information they may be able to dispute the chargeback directly with the card issuer. The dispute process can take up to 75 calendar days depending on the credit card issuer. In the meantime, the payment will show as reversed in your account. If they win the dispute they will notify you and your PayPal account will be reimbursed.
You can resolve a chargeback by either providing valid postage tracking information, providing valid proof of a refund to the buyer or by accepting liability for the chargeback
There is all this information plus a step by step guide to protecting yourself on paypals site
It doesn’t matter that you have a expired card but if you have already spent the money received then you may end up with a negative balance, but you have plenty of time to put the money in when you first get notified.
From Paypals site it says this on negative balance:
Some reasons why your PayPal may be in a negative balance include:
Failed transactions made using your bank account
Buyers filing disputes on payments made to you
Credit card chargebacks filed on payments made to you
Claims for unauthorised use on payments made to you
For more information on the specific reason for your negative balance you can review your transaction history.
Resolving your negative balance
If you have added and confirmd your UK bank account, you can add funds to raise your balance to zero or a positive balance.
To add funds from your bank account, follow these steps:
Log in to your PayPal account at http://www.paypal.co.uk
Click ‘Add Funds’ at the top of the page
Click ‘Transfer Funds from a Bank Account in the United Kingdom’
Enter the amount you wish to add
Click ‘Continue’
Review the details and click ‘Submit’
Click ‘View the details of this transaction’ to see the expected clearing date for the funds transfer
Note: It will take seven to nine working days for the funds to appear in your PayPal account.
If your PayPal account is limited and you are not able to add funds to your account, you can send a cheque, banker’s draft or postal order to the following address:
PayPal Europe
P.O.Box 9473
Dublin15
Ireland
Please ensure you include the PayPal email address that is to be credited and make your payment payable to PayPal
I hope this helps you.