Can you provide insight on whether or not GPC can be used to purchase annual subscriptions to online software as a supply or service.
I checked the DOD Government Charge Card Guidebook for Establishing and Managing Purchase, Travel, and Fuel Card Programs, and purchasing software was not on the Prohibited Items List unless the software supports a covered telecommunications piece of equipment or service for a substantial component or critical service. Based on your description of your requirement, it doesn't sound like this prohibition would apply. The guide also had a prohibition for purchasing software developed or provided by Kapersky Lab. Assuming this prohibition doesn't apply to you either, I don't see any reason why you cannot make the purchase for the service as long as it is under the micro-purchase threshold.
I think this is further reinforced by the policy stated in DFARS 227.7202-1 with regard to the purchase of commercial software licenses: "Commercial computer software or commercial computer software documentation shall be acquired under the licenses customarily provided to the public unless such licenses are inconsistent with Federal procurement law or do not otherwise satisfy user needs." In addition, there is not a specific contract clause for the purchase of commercial software (see DFARS 227.7202-4) which means purchasing on a credit card without a contract should be okay assuming this is customary in the commercial marketplace. FAR 12.212 states something similar: "Commercial computer software or commercial computer software documentation shall be acquired under licenses customarily provided to the public to the extent such licenses are consistent with Federal law and otherwise satisfy the Government’s needs." However, I would still check with your legal office, Communications Squadron and A-OPC on the local policy for this type of purchase.
With regard to your question about whether or not this is considered a supply or service, I could not find any guidance in the DOD Guide nor FAR/DFARS. However in your case (assuming this is a micro-purchase), I don't think there is any need to declare the acquisition of a license as either a supply or a service.