Shops selling wine/alcohol at Manila airport T1 and T4

There are, at least the outside ones, that are open until 21PM as far as I remember, do not know about the internal ones to the airports. For those outside, you have to get outside and catch a taxi. You can only enter on such shops showing your travel airplane tickets. However, those duty free stores are only for international flights, I am afraid.

As you are mentioning domestic flights, as inside the Phillipines, the prices are much cheaper outside in supermarkets than in the shops inside the terminals. They are all (mostly) upscale/luxury stores, and prices are more expensive on the shops inside the domestic terminals, to the tune of 3 times more expensive, at least for local goods for selling to tourists.

Thus, I would advise buying the bottle in a supermarket and not in the airport.

If you are worried about breaking it in your backpack, and we are talking about Port wine, you have got them in plain bottles, or bottles inside carton cases, metal and wood cases.

While you have got an added cost with the case, it still will be cheaper than buying it in the airport. It makes it more presentable as a gift too. (a wood case might also give you less problems with the x-rays in the airport than a metal case).

PS. I did not find the prices cheap in general in Philippines even in the international tax free shops. I did much more better deals in Istambul.