Visa Expired 4th April – Overstay/Immigration Tomorrow for sixty day Non-O extension?

Hi hopefully someone can give me some advice on my situation. I was getting my visa documents ready for going to Savannakhet tomorrow night for a Multiple Non-O based on marriage when, to my horror my visa expired on the 4^th, not the fourteen^th as I had down in my calendar.
Not wanting to chance getting stopped travelling as sometimes police check buses/vans, I now plan to go to Immigration tomorrow with the life companion and apply for a sixty day extension, then go to Savannakhet next month.
I see this as the most sensible/only option available to me and I take it I’ll have to pay 2500 Thb fine plus extension fee.
Any advice on this? Is this the correct way of going about this?
Slightly worried.