I agree that using 'through' is better than using 'by' in this sentence. Here are some suggestions: If you arrange your holiday with a travel agency, it will save you from the preparation tasks and you won't have to worry about looking for a hotel with all the right facilities. I would probably say: If you use a travel agency to arrange your vacation, it saves you the hassle of making all the arrangements or trying to find the perfect hotel with everything you are looking for. I hope that helps! -Amy

I would rephrase this statement even further to make it sound more natural and say: If you Book your Trips through a travel agency, they will take care of all the arrangements for you including hotels.

No that's incorrect. I would say: If you arrange your holidays THROUGH a travel agency you'll be free from preparation tasks and won't have to worry about looking for the hotel with all THE facilities. Or more natural: "If you book through a travel agency, you won't have to prepare anything yourself such as finding a good hotel" Hope it helped :)