I did the same , but I only overstayed 5 days it did not worry immigration they quite happy to accept what was it 500baht a day fine. Then they backdated the new visa to when my old one expired.
I only go Thailand 2-4 months a year as tourist so dont know how ever I cannot see how hard it would be to apply flll out do the form yourself. If you make a mistake fix the mistake and resubmit will get it right eventually.
A friend of my wife wanted visit Australia as a tourist. She went to a visa agent they told her she needed regular deposits in her bank account did not matter even if we guaranteed her stay.
I am not technically minded in fact stupid but why not buy a phone with 2 sim cards one with a Thai number the other with your USA number sim. Now when want to check send msgs, change to USA number switch to roaming in some place with WiFi. Can pick up a dual sim phone about $200.
I had overstayed my visa by 5 days, gave me a fine extended my visa for 30 days backdated to when original visa expired. It only 5 days though and I was at immigration office when it was picked up applying for a extension. The woman at immigration gave a little laugh at my explanation all good. I don't know about 15 days though pushing it a little .
I once brought in 4 litres of wine max aload 1 litre. I fronted up to customs showed them . They sent me on my way suppose paperwork to much trouble to pay duty. Food be different i would think are you willing to take chance of customs not doing a random check.
Have been told the International Permit is good for only the first 3 months on arrival even though it is dated for 12 months. I would assume if leave and go a country the 3 months start again. Why would you want to ride a bike in Thailand have been riding for 50 years to scared to ride over there.
I travel Thailand every year for the past 16 years. I always take travel insurance in fact am over insured I take out the free one with credit card and one I pay for..silly I think. I cannot fathom people who do not take out insurance. You always hear of stories people do not have insurance inevitable they have accident then they have a Go Fund me page to cover costs I have sympathy for them but do not give. My self believe should be a charge 500, 1000B whatever for every arrival into county or any country. It would more than cover costs to that country in fact make money. The insurance companies would not like.