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There is no way to buy app in the Microsoft Store in bulk. A workaround is that you could generate promotional codes for your app and give these codes to the customer. The customer could pay for these promotional codes to you in third-party ways and then you could give the customer the codes. After that, the customer could active the app with the codes in different devices.
There are something that you need to notice.
1) In any six-month period, you may generate up to 1600 single use promotional codes, or any number of multiple use codes such that the total allowed redemptions does not exceed 1600.
2) The 6 month period begins when you generate the first promotional code is created and lasts for 6 months regardless of whether or not you set an earlier expiration date on the codes.
3) Any codes created during an existing six-month period will be count toward the number of codes generated within that period, even if they will expire after the period ends (For example, if you generate a code on the last day of the six-month window, it will be will be still be valid for a full 6 months from its creation.)
More detailed information here: Generate promotional codes.
Thank you.
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