Shopify needs to warn merchants that this fraud is occurring. Whether the fraudulent activity is happening in Shopify's or UPS's system is irrelevant. The UPS accounts where these fraudulent charges are occurring are managed by Shopify and we don't have direct access to them.
To all merchants receiving UPS invoices associated with your Shopify stores, please take a look at the following threads regarding fraudulent UPS invoices Shopify merchants have been receiving lately.
Double-check your Shopify order history to verify that the shipments listed in any UPS invoices are legitimate. There appears to be widespread fraud affecting many Shopify merchants.
Here are a few of the Shopify merchants impacted by UPS-fraud associated with our Shopify accounts: