I have received a charge of $**.** on my DFS account for a subscription renewal to Micr...

I have received a charge of $**.** on my DFS account for a subscription renewal to Microsoft Office ***. I no longer have the computer the the subscription was for and want to cancel the subscription and be provided with a refund. I don't have the service tag nor the express service code of the computer that hosted Microsoft Office ***. The only order number of the Microsoft Office *** I have may be *********. However I can't be sure that this order number is correct. The only other information that I have are my customer ID which is ************ and my telephone number - (***) ***-****. Since I have been billed, I have made contact several times with Dell customer service and have not been able to have them help me as they say they need the order number of the purchase and all the order numbers that I have provided have not been useful to them. I don't understand that if I'm being billed through MyDFS that I can't cancel this order renewal. Please assist.