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Go to product configuration (Settings->Products & Services->[Your Product]). In general tab click on "Enable ProRata billing"
Available Pro-rata settings
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"Prorata day" - setup day of month you'd like to have your invoices generated on. i.e. "1" - will generate invoices for Monthly cycle every 1st day of month
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Customer purchased product on 13th of month:
- Customer is charged only for days he will use package this month
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