In my experience, Chilean customs officials are more than reasonable, especially with non-Chileans. Last year, as a foreigner resident in Chile, I flew from Toronto to Santiago with a bulky photo printer in a colossal box. Going through the X-ray check, a customs official flagged it and asked me how much it was worth. I informed him (correctly) that it cost approximately US$800, and that it was for a friend in Lima. He asked me when the unit was going to be shipped to Peru and I told him (again, correctly) that my friend was going to be traveling in two weeks' time and would take it with him. The official let me go, with no more than that verbal assurance.
My bet is that if you explain that the wine is going back with you to the US and show the official a return ticket, he/she will probably let you go. It will certainly help if you speak Spanish since most of the officials speak little or no English.
Of course, there are no guarantees! But Chileans generally are pretty relaxed about this sort of thing.
Still, I don't know why you wouldn't just take Chilean wine back with you. Santiago shops like Vinoteca and Mundo del Vino are adept at packing wine for shipping, and the Chilean plonk is as good or better (IMHO) than the stuff you buy in Mendoza.
By the way, you're aware that the bus ride to Mendoza is an eight hour affair with a potentially long wait at the border, and that in October the Los Libertadores pass can be closed temporarily on short notice, due to late winter snow storms?