As you know, something which is too good to be true is, generally, not. Also, I am sure you suspect this is one of those times. Unfortunately, I am absolutely sure that there is no prize waiting for you from these people.
This is positively a scam. It is known as an "Advance Fee Scam".
1. You can not win a sweepstakes or lottery you did not enter.
2. No legitimate sweepstakes asks winners to pay upfront to release the winnings.
3. If they were genuinely holding money belonging to you should be able to tell them to just deduct the taxes, insurance, courier fee (all red flags, by the way) and send you a net check.
4. No legitimate lottery or sweepstakes notifies major winners by text, email or regular mail.
Do not send any money or otherwise respond. They are trying to steal from you.
Here is an online warning from the FTC:
I realize this is probably not the answer you were hoping to receive. Also, please remember that this is not necessarily a moral judgement on my part. As a professional, however, I am sometimes placed in the position of having to deliver news which is not favorable to a customer's legal position, but accurately reflects their position under the law. I hate it, but it happens and I only ask that you not penalize me with a bad or poor rating for having to deliver less than favorable news.