You can make an electronic payment at the IRS web site using several methods, in which case you don't need the voucher. https://www.irs.gov/Payments
If you want to mail a paper check, the voucher should be included if you download or print the PDF for "all forms and worksheets." If you mail a check you may want to use FedEx or one of the US mail options that includes proof of mailing and tracking.
The interest and penalty formula is complicated, the best thing to do is wait for the IRS to bill you. If you filed on time and only paid late, the late payment penalty is 1/2% per month.