The table changed in v2020.  Primary Key now includes the Country Code too.

Are you suggesting the following?  

object obj = Mas90Plugin.Instance.ProvideX.NewObject("SY_ZipCode_bus", Mas90Plugin.Instance.Session.Object);

using (BusinessObject syZipCodeBus = new BusinessObject(obj))
{
    int result = syZipCodeBus.SetKey(address.Zip);
    if (result == 2)
    {
		result = syZipCodeBus.SetKey(string.IsNullOrEmpty(address.Country) ? "USA" : address.Country);
		if (result == 2)
		{
			syZipCodeBus.SetValue("City$", address.City);
			syZipCodeBus.SetValue("StateCode$", address.State);
			syZipCodeBus.SetValue("CountryCode$", string.IsNullOrEmpty(address.Country) ? "USA" : address.Country);
			syZipCodeBus.Write();
		}
    }
}

By the way, I'm not the original developer of this code.  Any links to developer documentation and/or examples of integration using ProvideX and c# would be super helpful.

Thank you.

It's not C#, but should help... scripting guide:

https://www.sagecity.com/support_communities/sage100_erp/f/sage-100-personalization-customization-and-productivity-tools/98736/scripting-helper-documentation

Look at SetKey and SetKeyValue.