Build or modify online shop

I build sites but am not a PHP programmer. Have tried various e-commerce software. They all need some customization in the checkout process. I can do the graphics, but the PHP is what I need help with. Checkout process is the most important feature. I sell digital downloads and physical products. 5 downloadable products, 4 physical products. For downloadable products: guest checkout. the only required field is their email address. nothing more. go right to payment. don't show fields that don't apply. For physical goods: guest checkout. require only the name, address, email. don't ask for a separate billing address. Not a big budget on this. Sorry guys. That's why I've been trying to do it myself. If someone can help me out at a reasonable price, I would love to hear from you. Note: there are things I like about Open Cart and PrestaShop. A modification of those would also be OK.