Is Dana (donation) more about detachment from one's possessions & to break one's ego or is it more about helping others? Whether it is booking a flight ticket, hotel room, buying an electronic gadget etc there are so many online stores which offer so many deals, that I spend a good deal of time finding the right deal, before spending. Furthermore, where I belong its common to haggle with shopkeepers to get a good deal.

And then, I occasionally donate money to some cause as Dana. 

That makes me think, what is the real purpose of Dana? Is it more about detachment from material possessions and sharing one's merits/ earnings with others to dissolve the ego or is it more about helping those in need? If it is the former, I might not as well worry about haggling with a shopkeeper or try finding a good deal online, and buy things at first sight, with metta to the seller considering I have donated the amount I could have bargained for to him. If it is the latter, I continue finding good deals and then donate the saved up money to those "I feel" (very subjective) are in need. 

So, how can Dana be explained in this context?
DANA SUTTA

In this Sutta, Buddha broke it down to 7 categories of why people give Dana 
(1) To answer your question as to purpose of dana, people give for many reasons. 
(2) some give to detach(ornament of mind) which bears great fruit and great benefit. 
(3) some people give to help others. 

I think you can answer yourself if the reason you want the merchant to maximize his profit is because you want to help him out or you are doing it to let go of your attachment.