Have you tried contacting your loan officer?
My best guess, based on the information you've provided, is that when you first spoke to someone several weeks ago, they locked in your rate for a specific amount of time. This is fairly standard with any mortgage and generally assumes that the loan will be closing by a certain date. If the loan didn't close by the date specified and rates changed, they would need to extend the lock in period or start over when you're in a better position to complete the process.
Additionally, I'm going to assume that the $400 was for an appraisal. That's based on the fact that every time I've paid for an appraisal in my life (half a dozen or so) it cost around $400 (give or take $50).
The bad news: If I'm correct, you can't get your $400 back and the bank probably has good cause to charge you the $150 per week to continue the loan process.
The good news: If it's going to be a while before the weather allows your contractor to do the work, the appraisal doesn't need to be redone (this next part is important) assuming the comps listed don't age beyond six months between now and then.
What you should do
Don't assume I know what I'm talking about. I'm guessing based on past experience and what you've told us. Call your loan officer at your earliest convenience and verify what you've paid for, what you will be paying for (if you elect to pay the $150) and what you will have to pay for in the future if you don't pay for the extension.
Whether or not you should pay the $150 per weeks depends, primarily, on two factors.
The weather: If you live in Northern Montana and it's not going to warm up for another 2 months, I'd consider letting it lapse. If you live in South Carolina, it's above freezing today so write a check and call your contractor.
Interest rates: I don't know what they're doing right now but your loan officer will. If you have a vague target of when the work can be done, he or she may be able to tell you what they expect rates to look like at that time. Based on that, you can make a better decision as to whether or not the $150/week is a worthwhile investment to keep your current terms intact.