Help with customising our Music Opencart Webshop

Hi, We are an independent record label and are currently rebuilding our webshop using Opencart. Here it is in it's current design stage. [login to view URL] We have chosen a theme we like and need to adapt it so that we can sell mp3 files along with the usual physical t-shirts, vinyl, cds and posters. We have a mp3 player installed for previewing the songs but we need help with adjusting the opencart store so that we can allow for file downloads and a quick way to add each song from an album individually to a shopping cart as well as the option to add the whole album download. Much like our ancient current webstore allows: [login to view URL] We purchased this extension for file downloads but can't seem to get it to work and we're not sure it's the right thing for mp3/file downloads. [login to view URL] We also need to make sure users only see the total price for each item e.g. 99p or £10.00 rather than a + / - of an original default price and can then add each item to the cart with a click of a button rather than the current checkbox style. Please see attached file with my crude hand drawn examples. We purchased this extension to to get rid of the prefix but we would prefer buttons rather than checkboxes. [login to view URL] Help also needed with how to update our postage costs to the current UK royal mail prices, sizes, weights etc. Since we have a lot of music to upload we would like to have some kind of easy to use bulk upload tool. Many thanks Amr