Gift Basket builder script

Hi,

I'm looking for a script, or someone who can write a script, that lets me build a gift basket or hamper from a selection of products on a web store site.

For an excellent example, check http://www.scottishhampers.co .uk (minus the space)

Here you select a size of gift basket, and then, I presume based on some custom attribute on the product which gives it a relative size, it "fills" a counter up to 100% until your basket is full and can fit on no more products. For example a small basket gives you space for, say, 8-10 jars, or 5-6 jars and a bottle of wine.

Delving into the source code for that page there's a "byo.js" script running which seems to determine this behaviour. I would LOVE to know how they did this. It looks custom-written to me, but I may be wrong. One of the other scripts on that page was called "slideshowbyo.js" which linked back to this site as the source, hence my posting here. It looks like they're working together to produce that cool dynamic percentage full meter.

Does anyone know if this kind of thing is available as an add-on, free or commercial, to any of the popular ecommerce CMS's out there? Their forums are pretty useless and surprisingly Google has let me down as well...too many unrelated search results for gift shops and the like.:p

Thanks in advance
Lee