I asked tech support, and this was their response: Thank you for contacting Merchant Technical Support. As described, proration is not supported. If you need to update pricing to adjust for this, please follow https://developer.paypal.com/docs/multiparty/subscriptions/customize/update-plan-pricing/ or https://developer.paypal.com/docs/api/subscriptions/v1/#plans_update-pricing-schemes. If you want to collect a single payment outside of this subscription, you can use invoicing. Thank you for your business and I hope you have a great day! So invoicing is their solution to this problem. I looked at the documentation for invoicing and it looks like it could work, but there is going to be extra steps and won't be as elegant as PayPal supporting proration. Hope this helps.
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