To send your PayPal Customers to ConvertKit, you'll need to use Zapier.
Please note that using PayPal with Zapier does require Zapier Premium, but there is so much you can automate with Zapier, beyond PayPal. Check out other recipes here.
Here's the one-two punch to set it up!
Select the Trigger "Successful Sale" in PayPal.
Select the Action "Add a Tag" in ConvertKit.
Turn on your Zap and Voila!
For multiple items in PayPal:
There are two successful sale options. Choose the one that says "with line item support" if you want to set it up for multiple products.
You'll also want to add a filter to your Zap, so you can filter by a sale of a specific product.
Click the plus sign between your trigger and action, then choose "Filter" from the righthand column.
In the first drop down menu, select "Item Names" from PayPal, and choose the correct field. In the second menu, select "(Text) Contains." In the last box, write your item name exactly.
Create a zap like this for each of your PayPal products!