Hi J.Smith@FLTacademy , 

Thanks for reaching out. We have a listing of 3rd party solutions available on Sage Marketplace here for Sage 50 US (you'll need to select the product using the filters).

Additionally, while not directly related to upfront payments or Proforma Invoices, Sage 50 US Edition (2015.3 and higher) offers enhanced features for applying Sales Orders to Sales Invoices, which could streamline your billing process once a prepayment is secured. Here's a brief overview:

• For Sales Orders: Navigate to Tasks > Sales/Invoicing, select the customer, and apply the sales order number. This action pulls the sales order items into the invoice, which can be finalized and saved.
• For Purchase Orders: Go to Tasks > Purchase/Receive Inventory, select the vendor, and apply the purchase order number. Similar to sales orders, this action brings the purchase order items into the invoice for completion.

These features may assist in managing the flow from sales order to final invoicing, ensuring a smooth transition once prepayment via your Stripe integration is processed.

Hope this helps! If you have further questions or require additional assistance, feel free to reach out to our support team or visit the Sage Knowledgebase for more detailed guides and articles.

Warm Regards,
Erzsi

You can do what you are looking for but just can't with your current configuration.  I have other clients doing exactly what you are looking to do by using Pay Invoice instead of Stripe, you will then have a payment link to send with the sales order or proforma invoice and when the customer pays it will come back into Sage automatically as a prepayment under the customer record and then you can invoice your order and apply the prepayment.  You will most likely also save on fee and level 3 processing with Pay Invoice as many other clients have.  Reach out directly if interested.  [email protected]