I don't think YABU but there will be the usual contingent making you the wicked step mother.
New clothes for a holiday is fine (though the designer brands are imho excessive), the problem here is the wider context.
Part of good parenting is teaching that actions have consequences.
What she's doing is essentially using her father to buy new things and score a holiday, whilst simultaneously refusing to have any relationship beyond one which is rude and disrespectful.
By allowing this she is learning that treating people in this way. It's not.
She behaves like this because your DH lets her and one day he will wonder why the hell his DD has grown into a selfish, self absorbed adult.
Personally I'd be happy to give her a reasonable budget for holiday clothes which she can source and buy herself on the basis she quits the rudeness/stroppy texts.
I'd also be clear that she's expected to behave on holiday and be polite. If not she won't be invited again.
Re: contact I'd give more leeway and I would not make this an issue about being able to go on holiday. At 15 she'll want to hang out with her mates rather than family.