Per IRS Form 4868 instructions, "If you e-file Form 4868 and mail a check or money order to the IRS for payment, use a completed paper Form 4868 as a voucher. Please note with your payment that your extension request was originally filed electronically." That same document gives the following address for mailing an extension payment when you live in Utah:
Internal Revenue Service
PO Box 7122
San Francisco, CA 94120-7122
