PhotoStore Documentation

The digital profiles my customers download are 96DPI while the original I uploaded is 300DPI

he digital profiles my customers download are 96DPI while the original I uploaded is 300DPI?
That is correct, if you have your store setup to automatically create digital profles and the customer downloads one it will be in 96DPI.

Why can't the store do 300DPI on the digital profiles?
The PhotoStore can do 300DPI but in order to do it you must log into manager and go to settings -> software setup -> image processing -> and set the store to use Image Magick as the image processor. If Imagick is grayed out then it means your hosting doesn't have the Imagick module installed as a PECL for PHP and therefore the PhotoStore can't use it. By default the PhotoStore uses GD Library which almost all hosting do have installed, and that GD Library is only capable of doing 96DPI when it makes a photo. If you use Imagick the digital profile sold will be the same DPI as the original media you uploaded.

If I can't enable Imagick is there another option for my digital profiles to be in 300DPI?
You can manually attach a 300DPI sized profile to a media. To do this go into your manager and click on library -> digital profiles -> create a new profile or edit an existing one and set the delivery method to "attach file". Then go to library -> media and click on the media you want to edit. Go to "digital versions" tab, and in there scroll down to where it list all the digital profiles for sale on that media. Put a check in a box for a profile you wish to sell under that media and then click on the "attach file" button to attach a file to that digital profile on that media. When sold the store will give that attached file as the download instead of creating one.
You could also just set the size  (width & height) larger to compenstate for the 96DPI. That way the end user can download a large photo and then resize it to 300DPI and it would be the size you would of sold it at if it was 300DPI to start with.
There are a lot of websites out there that talk about calculating sizes and you may want to research to see if they can help you determine the sizes you need to offer.