To add multiple contacts to a vendor, go to Orders > Manage Vendors tab in the navigation bar. Select a vendor, then at the bottom of the Vendor Details that pops up, you can add or delete contacts. More info on editing and adding vendors here.

When reviewing your orders on the Shopping Cart page, you can choose which contacts to send your orders to by selecting or deselecting the Email Order checkbox next to their name. The same email selection boxes appear just before order placement on the mobile app as well.

In cases when a contact is out of office, or their email bounces, you can resend the order to another contact by clicking on the specific order on the Order History page, then selecting Send order to.... within the Actions menu. Select the vendor contacts you want to send the order to or enter in an email address. Add multiple addresses by clicking the +  button, then Send. For the mobile app, tap the three dots to bring up the Send order to... option in the order details header.