How do you pay extra for win 7 support for one year?

You contact a Microsoft Authorized Partner - most consultants who work with Windows are.  If they can sell Office 365 they can sell you the additional year of updates for Windows 7.

Indeed, where XP was an option for big Enterprise customers, now you can buy even just 1 license through an MS authorised reseller. Though the advertised pricing on many sites (50 bucks) is not correct, it's a tad over 70 bucks.

Side note:  Microsoft does not intend this to be a long-term project.  What I've read says the price will double every year; then, at some point MS will cut it off regardless except for government and large corporations.  The intent is to stabilize mission-critical existing W7 systems enough that eventually the price will be prohibitive and they will be migrated to W10.

Year 1:  $50
Year 2:  $100
Year 3:  $200
Year 4:  $400
Year 5:  $800
Year 6:  $1600

Actually, it's planned to stop after year 3.

when you pay for the 1 year of upgrades, what do you get? A MSI file that you click on? how do I get this to the customer who is not an O365 user? I buy it from a MS reseller, but what it’s it and how do I tell the computer it’s Ass is covered for 1 more year ? :-)