In QuickBooks Online, mailing a check is unavailable. Though, you can create and print it.
Here's how:
Click the Plus icon (+) located in the upper right-hand corner.
Under Vendors, click Check.
Choose the Payee from the drop-down list.
Specify the Bank Account where the money will be withdrawn.
Complete the check fields. (Note: The Memo field is optional. Text you enter in the Memo field appears in the register (or account history), on printed checks, and on reports that include this check.)
Check the Print later box if you want to print the check.
Click Save and close or Save and new.
Please let me know if there's anything else you need help with QuickBooks. Thanks.