Here's a snippet of life outside the customs union. I'm getting something forwarded from Germany using a forwarding service, and here are the postage prices charged by UPS:
https://www.mailboxde.com/united-kingdom.html#ups
To send a 10 kg parcel from Germany by UPS to other EU countries it costs between 13.49 and 18.01 euros depending on distance. To Switzerland and Norway, inside the single market but outside the customs union, it costs 29.72. It costs more than twice as much to send to Switzerland, a country bordering Germany, than to the UK.
https://www.mailboxde.com/united-kingdom.html#ups
To send a 10 kg parcel from Germany by UPS to other EU countries it costs between 13.49 and 18.01 euros depending on distance. To Switzerland and Norway, inside the single market but outside the customs union, it costs 29.72. It costs more than twice as much to send to Switzerland, a country bordering Germany, than to the UK.
David Cameron and George Osbourne said multiple times that a Leave vote would mean leaving the Single Market