Are there different options through which one can buy a subscription?

Yes, Microsoft offers multiple ways by which one can buy Azure subscriptions. On a high level there are three channels

Enterprise agreement – For medium to large enterprises looking at making a pre-commit on the amount of Azure resource consumption. This is typically a Pre-Paid annual payment.

Open License Program – For small and medium organisations who would choose to use a Pay-As-You-Go model. Credits can be added through activation of open license key bought from Microsoft resellers. Also available through web direct options, purchased through Microsoft websites.

Microsoft Partners – For small and medium organizations who would choose to use a post-paid Pay-As-You-Go model. This is available through Cloud Solutions Providers – CSP Partners.