You will have to mail in the payment with your return. From the 502X instruction you don't need a payment voucher for your payment:
MAILING INSTRUCTIONS
Mail your return to:
Comptroller of Maryland
Revenue Administration Division
Amended Return Unit
110 Carroll Street
Annapolis, Maryland 21411-0001
PAYMENT INSTRUCTIONS
Make your check or money order payable to “Comptroller of
Maryland.” Write the type of tax and year of tax being paid on
your check. It is recommended that you include your Social
Security number on check using blue or black ink.
<a rel="nofollow" target="_blank" href="
https://forms.marylandtaxes.gov/17_forms/502X.pdf">https://forms.marylandtaxes.gov/17_forms/502X.pdf...>
