The simplest way to do this is to use www.paynow.co.zw. you can integrate it with your entire e-commerce website or set it up for simple one-time payments. Paynow allows you to accept payments from Telecash/Ecocash/Mastercard/Visa/VPaymens(zimswitch).
However the downside is that you can't receive these payment directly into your Ecocash. The payments will be channeled into your bank account which you set up initially. You can setup multiple bank accounts.
The way I see it you are better off with a Steward Bank account linked to your Ecocash such that you can easily withdraw money to your Ecocash from the bank account via mobile phone and use your Ecocash Mastercard.