You can hang a sign outside your door.
No. I don't think that'll work. You've got an online store, you can't use old school advertising because your missing 99 percent of your customers. You put a poster up. Well everyone who walks by the poster will see it,. Or, you use google ads, or facebook, and 1000's of people see it.
The marketing for big brands is different. For big brands it's not as much about building a customer base, it's more about keeping a customer base. So they don't want to take a risk. The internet is a risk, because they can't control the content. For example, you put an ad up on google and your ad could end up next to an article about Hillary Clinton. You do that a few times and now customers will have the same feelings about your product as they do about Hillary Clinton.
Or you'll almost never see food advertised on the internet. Because the last thing they want is their food playing right after someone vomiting, or doing something disgusting.
Big advertisers like TV, because they can control the content. TV shows work it so that you have good feelings about their commercials.
For a startup company, advertising on the internet isn't such a big risk.