Personally I wouldn't use it - if you can't pay on time you will be in trouble if you have an IVA as it can be seen as credit. Think about using Paypal or just your normal debit card.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
If whatever you are purchasing is over £500 that is a no-no. These short term loan firms also tend to be more aggressive if things do go wrong. If they credit check they might well refuse you anyway.
The generally included clause that you cannot take out credit more than £500 without the IPs permission could also be taken to cover things like monthly pay insurance (often a loan) and utilities on credit meters --- it is easy to exceed that £500 without adding things like short term credit purchases.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk