Do you want to just have your single server update it's own free certificates using Let's Encrypt? Download RenewCertificate.ps1 and add as a scheduled task to run every 2 months! That's right, Free Automated Certificate Installation.
After downloading the powershell script, update the following parameters:
$name = "NAME" # This is just a friendly name used as the site identifier by ACMESharp
$site = "Default Web Site" # This is the IIS Site Name that has the bindings you need
$dns = "example.com" # This is the root dns name
Add your aliases!
$alias += "www.example.com"
$alias += "ftp.example.com"
$pwd = Generate-Password # or don't and use something you know.
PFX files will be stored in the same directory that the script gets run in.
If you've never used ACMESharp, don't forget to initalize the vault and agree to the terms of service! See ACMESharp for more details.
PS:> Import-Module ACMESharp PS:> Initialize-ACMEVault PS:> New-ACMERegistration -Contacts mailto:firstname.lastname@example.org -AcceptTos
To set up a scheduled task, create a batch file that runs the script.
Open Task Scheduler
Create Basic Task...
Set Monthly Trigger to every other month
Action: Start a program</br/> Select your batch file