Sentence:  If you know what you want by filtering down the 1,600 or so trusts to a short list and then studying the information on each via online sources, then you can buy your trust and get all or nearly all of the commission back into your bank account via an Internet investment <WSD>supermarket</WSD>. Here, you select your fund, decide whether you want it as an Individual Savings Account (ISA), and pay for it with a credit card. 
Meaning of the word marked with <WSD> tags:  A one-stop shop; a place offering a range of products or services.