Hi leahk,
I have done a replica test of the process of booking this holiday, and I can't see any problem. There is a little circle next to each option, and underneath the 'pay infull' option, you should see 'pay deposit' with this circle next to it. If you click on this circle it will select the ‘pay deposit’ option.
If you are still having problems, you could always speak to one of our friendly advisors on 0844 273 8256.
I hope this helps.
Kindest Regards,
Monty