You can purchase any external services which contribute to your Estonian company's business activities. Examples include:
Marketing and consulting
Business, IT, legal and tax advisory services
Accounting and administrative support services (e.g. Xolo)
Hosting services (e.g. Amazon, AWS)
Domain, trademark, and patent registration
External certification and auditing services
There are no limitations to the origin of the service providers, or to the number of services purchased per month/year as long as your company needs the services, and has sufficient resources to cover the costs.