In some places you can apply for the extension 30 days before the expire date; I think 15 days in advance is to play safe in almost all inmigration offices.
The same with me: I'm slowly traveling through SEA (it's my 3er visit, previous were on 2013 and 2014) and like to stay for some time in Thailand because it's the country I like the most :)
I'm not doing visa runs/bounces, in fact I'm visiting all SEA (Laos, Cambodia, Thailand and now Vietnam) but staying in Thailand between countries.
In addition, my onward travel was bought back in December - I bought a return trip because it's cheaper and because I need to return to home in summer :D
Rick, I've read all the e-ticket for 5 or 6 times, and there's no one line saying I must show the card. The same said the operator who atended me by phone.
I hope it's just the onward what they want to see.
Rick, thank you for the tip regarding the credit card. I've just phoned the company and had confirmed me that there's no need to show it - I bought the tickets straigh from the Cathay website, not agents. The operator confirmed me that is 99% sure they want to see proof of onward travel.