Now if programmer has to be around to update the rules, they are best written in existing language for which you can easily get somebody who understands it. For example PHP.
Now to the point of hard coding. PHP is not a compiled language. It is not even precompiled. The interpreter actually reparses it on each request. So when you change the file with business rules definition, it will immediately take effect. You want to version that file separately from the core of the application or you may even store it in the database, but PHP is probably simplest to implement, quite simple to maintain and also fastest as any rule interpreter written in PHP is going to add noticeable overhead.
You may want to use some sandbox (quick google reveals at least two implementations for PHP) to ensure that the business rules file can't touch anything outside the transaction object. This can be added anytime later, you can start without bothering about it. You may also later add some graphical language that will generate PHP into the database or something, but it's been repeatedly shown in practice that it takes a programmer to edit nevertheless, so it's effort with questionable return (though it might be a selling point).
(Edit) As I looked at decision tables, they don't seem to be very flexible. They need a column for every possible criterion, so as the rules grow in complexity, the table will need more and more columns and it won't be easy to understand any more anyway.
Also don't forget that an e-commerce package does not live in void, but needs to be integrated with various other systems. Most shops will probably already have a accounting system where they need to export data about finished transactions and store management system where they need to export data about goods to ship or in case of software shops accounts to create and licenses to assign, they will be using different payment processor to which you have to redirect and receive payment confirmation, etc. For these the logic will be different, so simple rules won't cut it. You absolutely need to support writing this in PHP, independent from the main core just like the business rules you mentioned.