Voluntary class 2 contributions for self-employed

I have done some self-employed gardening over the years but never reported the income to HMRC because it was always under £1000. Now I understand that self-employed people can make voluntary class 2 NIC contributions if they want to, and these are much cheaper than class 2. Going forward, can I do this? What if I have a year where I make £100 gardening? Or even £0? As long as I have registered my gardening services as a business, am I eligible to make class 2 NIC contributions? And if I am, can I file back taxes for the years I didn't do this?

Hi Erin, 
From April 2015 you can pay Class 2 voluntarily, via your Self Assessment return, if your profit is less than the Small Profits Threshold (SPT) up to 31 January following the year of the return.
If you wish to pay voluntarily after 31 January, you need to contact the NI Helpline 0300 200 3500 to arrange payment.
These contributions need to be paid before the end of the sixth year following the end of the tax year in which they were due.
If you do not have the option to pay voluntary through your tax return, you may only be registered for self assessment and will need to register as self employed.
You also may have reached thresholds which treat your contributions as being made and you will not need to contribute.
If you have past tax years where you were self employed and did not pay class 2 contributions you need to discuss with National Insurance to see if you are eligible to make payments.
You can call National Insurance on 0300 200 3500 or use the webchat service ‘ask HMRC online’ through gov.uk. 
Thank you.