If you provided accurate measurements and instructions, you should not be left out of pocket when the vendor has to fix something which was his fault in the first place. So yes, you should demand that he refunds the �30.
However, if the vendor manufactured the cage to your specifications (i.e. there was no error on his part) then he's doing you a favour by making alterations free of charge (and even more so if he's not charging you for the additional carriage he'll have to pay to return it to you). He's waived his bills for labour and carriage, so it would definitely be 'cheeky' to ask him to pay for your carriage as well!
So, you need to decide who was to blame for the cage not fitting properly. If it was the vendor, demand a refund of your �30. If it was you, say nothing or he might send you a bill for his additional working time and/or the second lot of carriage charges.
Chris