The delay they refer to, is the accounting of the data you used whilst in Thailand, not the SMS being generated itself.
If you as you claim have incurred $4000+ in excess charges, this means you've used more than your travel pack allows. And given they send an SMS at every $100 of spend, I would expect you to have received 400 SMS alerts. Because you only get 100mb per day of pack you choose, so if you buy 7 days worth of packs, then that's 700mb. Now in Thailand, data is $1/mb (billed in 10kB increments). This means to get a bill in excess of $4000, you've used nearly 30gb of data whilst roaming. (Please note this is really rough math, but if your consumption allegation is correct, then you're using a TONNE of data).
When you say "You purchased the pack", did you go online PRIOR to your trip to activate this? or during your trip and specify the said days?
I've got a few ideas, but there is a lot of holes that need to be filled to make a fair judgement of the situation.
Everything I've quoted from is in here: https://www.optus.com.au/shop/mobile/international-roaming/postpaid