Be careful when booking!! I booked a airport trans...

Be careful when booking!! I booked a airport transfer a month early and made an error with the time (chose am instead of pm). Emailed them straight away to amend the time and was told it couldnt be done. Unsure why this is; surely they could amend the time somehow? I'm travelling for a whole month to 5 different counties; another company can have my money i guess..... just be careful with booking!!