Replace Appointments+ ID with better description in pdf invoice

Hi

Please see attached screenshots for problem. Basically when I make an appointment on my site (using Appointments+) I have ensured the order looks like this

SERVICE, SERVICE PROVIDER, START TIME in the shopping cart

Unfortunately on the invoice sent by marketpress it shows as
SERVICE, APPOINTMENT ID

I don't want it to show the APPOINTMENT ID it's confusing and not much use. I want it to instead display as I'm setting it up in appointments. SERVICE, SERVICE PROVIDER and START TIME

Many Thanks
Kieran