Almost all destinations of the day tour have souvenir shops where you can buy "pasalubongs" or mementoes.
What to buy: You can buy old currency, stamp collections, postcards, t-shirts, caps, keychains and other mementoes.
What to pay: P100+ depending on your chosen souvenir item
There is a small souvenir shop at the Corregidor Inn if you are interested to buy souvenirs of the island. Besides the usual postcards, key chains, T-shirts etc, there is a book on the history of Corregidor Island called "A Galleon Guidebook - Corregidor XiXth edition by Alfonso J Aluit" whichs cost about 300 pesos (if you are interested in history).
A very important thing to note is that it seems this is the only place which you can buy souvenirs of Corregidor Island, so you can do it after having your lunch at La Playa Restaurant (part of the day tour package) or if you are stay here overnight.
Souvenir Items are all around the Island wherever you make a stop over to one sight to the other. There are lots of souvenir items you can get and they are offering different things also. Like for instance, in one store there are shells and old WW2 bills but most of the stores have mugs, caps and t-shirt.
What to buy: Our tour guide told us that when it's raining, they can dig old WW2 coins and they sold it for P100 per coin to the local gift shop in Corregidor. It's a nice souvenir. sometimes they could get dog tags also but the price is really high but if you are a collector of sort, you can find it there or ask the tour guide where to find it.
What to pay: Cash is the only mode of payment in the souvenir shops but i think you can pay your ticket at CCP by either card or cash.