I am totally ignorant when it comes to e-commerce software - but have you checked www.digistore.co.nz - I suspect you need someone who knows what they're doing to help install it - but as it's open source at least you have flexibility with it and they have an on-line forum where you can find people to support it. Just remember I have NO experience with this stuff - just like open source - plus this seems from the website to have a Kiwi flavour somehow - pretty unusual.

It's pricey, and they're trying to move people to a SaS (Software as a Service) model, but Miva Merchant still have a full version available if you hunt around in their online store under 'other products'. It's $995 USD but many people swear by it as far more flexible than open source solutions, and updates are certainly very straighforward, just point and click, although you need to pay an annual subscription if you want updates beyond the first year.
I've done a bit of work with Miva Merchant myself, and I know a few NZ hosting providers eg. Webfarm, provide it as part of a monthly package, although you said you want something to purchase. The price is the main thing that has put people off, but if the cost is acceptable, having something that's commercially supported and dead easy to update is maybe worth a look.

I thought OSCommerce was as good as dead... Buggy. You said you don't like monthly payments, but if you're serious then checkout www.bigcommerce.com Developed in Auzzie, but are the fastest growing sAAs ecomm platform on the planet with over 15,000 users.

The self-hosted open source stuff (e.g. OSC, Magneto etc) and those options supplied by local web hosts as 'free' can be a real nightmare. If you're a bit of a geek and plenty of spare time of your hands, then go for it. Otherwise save the grief and try the auzzies out for around a $1 per day. It has all the local banking gateways sorted out too. It just works....

Prestashop is fabulous! Some clients of mine have asked for Magento and Zen Cart but Prestashop is my prefered ecommerce system the back office is very straight forward and get the hang of.
www.prestashop.com oh and the forum is great for getting help if your stuck we are pretty much always around to answer your questions quickly.
Thanks, Jess
MommaBytes
www.mommabytes.com

This system is a full e-commerce system. It not only has a full e-commerce software but you get an autoresponder, crm system and affiliate capability. https://crm.infusionsoft.com/go/robotRP/hannahd

The above link posted twice for some reason.

I used ecommerce software solution by dotCommerce. They have very friendly team which is ready to help you at any time. View more about this company - http://www.dotcommerce.co.uk/

The answer is and always will be Advansys Such high quality design and a top notch team as well, highly recommended