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Each time a purchase is made on your Shopify store, ReceiptOK automatically produces a digitally signed PDF receipt and sends it off to your customer, immediately upon purchase.
ReceiptOK automatically produces receipts for purchases made on the Shopify platform without manually producing a receipt for each purchase. Furthermore, ReceiptOK uses digitally signed PDF files, which may satisfy requirements made by local tax authorities in the jurisdiction where your company is incorporated or domiciled. You must, however, consult a local lawyer or tax advisor to make sure the receipts produced by ReceiptOK actually do satisfy these local laws and regulations, which may also change over time.
ReceiptOK uses a digital certificate that uses PKCS#12 (Public Key Cryptography Standard #12) encryption to sign the PDF receipts that is produces. Each jurisdiction and tax authority has its own laws and regulations regarding the requirements for digitally signed receipts. Please consult a lawyer or tax advisor for more information about compatibility of ReceiptOK documents for your needs.
After you install the app on the Shopify app store, you should make sure your Shopify store collects customer email addresses during checkout. This requirement is important because ReceiptOK can only send receipts to customers it has the email address of. Naturally, if you do not collect your customer's email addresses, ReceiptOK will not be able to send them their receipt. For more information about changing this setting and making sure your Shopify store is compatible with ReceiptOK, please see the instructions here.
For ReceiptOK to be able to send receipts directly, you should set Shopify to collect email addresses from customers during checkout. ReceiptOK will automatically mark your settings as "Verified" once your store has one or more orders that were placed after the Shopify setting was set properly. This could mean, however, that your settings remain "Not Verified" even after you have set the Shopify setting correctly. Don't worry; once a new order is placed, your ReceiptOK settings will be marked as "Verified" automatically.

If your store is not set to collect email addresses on checkout, but the customer still provides their phone number, the receipt may be sent to them with an SMS text message, depending on your individual ReceiptOK settings.
A company domicile is the legal jurisdiction of a corporation and the place that's considered by law as the center of corporate affairs. It is typically the country or region where the company was incorporated. It will be included in the digital signature of your outgoing PDF receipts, similar in spirit to the country indication of EV (Extended Validation) certificates.