Oh and by the way, Google gave me this so far (not able to test, only later in the day):

One:
/script BuyMerchantItem(1);
/script BuyMerchantItem(2);
/script BuyMerchantItem(3);
/script BuyMerchantItem(4);
/script BuyMerchantItem(5);
/script BuyMerchantItem(6);
/use Commendation of Service

(After you've got the vendor window open)

Another:
/use Commendation of Service
/script local function buy (n,q) for i=1,100 do if n==GetMerchantItemInfo(i) then BuyMerchantItem(i,q) end end end buy ("Commendation of Service",1)

Feel free to test it and share your results ^^

Cheers