This is my opinion, so take it for what its worth, but I would personally go for a wormer where each chicken is individually dosed rather than one that's added to the food or water. Some chickens eat or drink more or less than others, so some may be getting too much and some too little.
If your feed store didn't have the Valbazen, you might look for Ivermectin instead. It can be applied to the skin (typically under the wings) like a spot on treatment for dogs and cats. Its a little pricey as well, but that bottle will last you quite sometime. If the Wazine is your only option, and you absolutely must use it, then do what must be done. But it isn't necessarily the most effective treatment out there. This is one of those deals where you get what you pay for, and there is a reason the Wazine is much less expensive than the Valbazen and Ivermectin.
Good luck.