@Alex **********
that is the eEXTENSION service offered by VFS global which charges a FEE depending on how fast you want your extension approved.
The cheapest price point is 500 baht PLUS the 1900baht for the extension). You apply, upload the documents, get the approval then you go to the eExtension office (which is NOT at IT Square) but out at the government complex where the main immigration office is) on the floor above it
READ the FAQ's on the link that San jumped you and you will see how the eExtension process works