The sales module provides insight into prospective customer sources, personalisation of service and customer engagement data, sales productivity and performance metrics on many platforms such as dynamics magento and dynamics shopify.
Looking through the dynamics integration, it looks like this doesn't allow only syncing specific contacts, but would sync all contacts between the two systems so this may not be suitable for your needs.
Dynamics 365 full licences start at 53.40, additional apps (attach licences) then start from 16.40. There are education, government and charity discounts for dynamics licences, and different apps have different pricing. Some are priced per user per month, but others are priced per tenant per month.
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