I have used ECV's at the ticket counter before, so that should not be an issue. You want to make sure that you have the number with you that was included in the email from Customer Care so the people at the counter can pull it up easily.

My assumption is that you have a $150 credit on the voucher...just a guess If this is the case then I do not see why they wouldn't let you use if for the pet fee. The voucher simply has to be used by person who's name is referenced on it, and cannot be used for a ticket for someone else.

Good luck!