which is the easiest to customize 3rd party shopping cart/ecommerce packages?

[lawrence k]

I know this question has been asked before, but every few months there
are different levels of activity on the various open source projects,
so I figured I'd ask again to hear what the current concensus is.

I need to implement a simple shopping cart for a company I work for.
They will be selling digital content (MP3s, videos, etc) online. I'd
like to use someone else's software, rather than write my own shopping
cart code. The shopping cart package should be:

1.) Written in PHP

2.) Open source.

3.) Does not hardcode HTML into the PHP, but rather, allows the easy
modification of HTML without having to edit PHP files. This should
apply to both the public pages as well as whatever control panel/admin
it has for the site's administrators.

4.) Offers modules that easily support the APIs of various credit card
processing companies (PayPal, Authorize.net, Verisign, etc). Hopefully
it supports more than one company.
Please let me know what you'd recommend.