The most common way in Universal Function – File Exporter (UFFE) to mark something as printed is with a SQL Query.
If you for some reason do not wish to use Update SQL to do this (Technically legal as it is UDF but a “gray” area), you can instead do it with C# and the SDK. The drawback of this approach is that it is slower but follows all official rules of the SDK.
The attached document shows you how to do it.