I hope you don't implement any discounts for past purchases! People buying that without complaining means they value your product highly. Their valuation of your product probably increased after they used their first pack.
You should take this into consideration and beef up your try-before-you-buy marketing - given the data, I think your bundle is probably supremely undervalued right now.
Coupons distributed to existing customers are quite effective. You have a list of people who are already very interested in what you have for sale and you know are willing to spend money for it. I have gotten offers plenty of times when I wasn't actively in the market for the goods in question, but then started browsing and purchased something. It's also a good way to keep mind share with your customers.
good advice, I wish to expand on it with my experience:
It doesn't matter if the clients are complaining, as long as they are buying.
It's customers' job to complain about something - usually the price - and if the price is the only thing they are complaining about, you're probably doing great :-) If nobody's complaining about the price, it is usually a sign you're selling yourself too cheap.
I always look at how the customer is actually behaving, not what he is saying. If they stop buying, then change something - not before.
You should take this into consideration and beef up your try-before-you-buy marketing - given the data, I think your bundle is probably supremely undervalued right now.