Have you priced the same trip with Costco, cruise411, and Dreams Unlimited? If not, check them out. If Expedia is really the cheapest, I'd think it would be fine to book there. There are reputable.
I booked a cruise through Expedia and had no problems. They had a special rebate promotion and I received my rebate after my sailing with no problem. I wouldn't hesitate to book through them again.
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