I paid $100 on the extended payment form but don’t see it reflected.

Depending on where you are looking, it may take a while to be reflected.

I would look at your IRS Account.

IRS Individual Online Account  

• Taxpayers with a Social Security number or an Individual Taxpayer Identification Number can create or access their IRS Individual Online Account to get information they need to file their return.
• With an IRS Individual Online Account, people can:
• View balance owed, payment history and schedule payments.
• Cancel scheduled payments.
• Get transcripts.
• View or create payment plans.
• See digital copies of some IRS notices.
• View key data from their most recently filed tax return, including adjusted gross income.
• Validate bank accounts and save multiple accounts, eliminating the need to re-enter bank account information every time they make a payment.
• View, approve and electronically sign power of attorney and tax information authorizations from their tax professional. 

Did you pay  $100 with an extension to file?  If your extension payment from 4868 line 7 did not automatically show up on 1040 line 31 you need to enter it. So make sure it's not already there before you enter it!

To enter the amount you paid with an extension (including state extension) go to

Federal Taxes (or Personal for desktop H&B version)

Deductions and Credits

Then scroll way down to Estimates and Other Taxes Paid

Other Income Taxes - Click the Start or Update button

Next page second section Payments with Extension

Either Visit All or Click Start or Update by the extension you paid