Sri Lanka visa for a 35-day stay: can I extend on arrival or pay for a 5-day overstay?

For a 35-day stay, one suggested option is to skip a visa extension and pay for the extra five days as overstay; the quoted cost is US$60.

Travelers touring the whole island may prefer to sort out an extension in advance so they do not have to return to an immigration department from a distant part of Sri Lanka.

The question specifically concerns arranging a one-month extension on arrival rather than paying an overstay charge. The supplied messages do not provide a confirmed procedure or location for obtaining that extension at arrival.