Shopify require the use of a business PayPal account:
Set up PayPal on your store - Common Issues.
Even more in their User Agreement, PayPal states that business accounts are for the ones that primarily use PayPal to sell goods or services, while personal accounts are recommended for individuals who shop and pay online, or wish to send or receive personal payments.
You can also use a personal account to receive money for the sale of goods and services, but if you plan to use your PayPal account primarily to sell things, you should consider a business account. You can upgrade a personal account to a business account should circumstances change.
AddTrackingInfo also requires a business PayPal account, as this is the only account type that allows creating dedicated sub-accounts (up to 200 of them).
If you've reached this point, you already know the reason. It's always better to cash in...