Mr J,
Online books will let you have any amount on the dogs BUT they only pay tote odds (normally Home State).
You could have $5000 on a dog with IAS but you can be sure that IAS will lay off on the tote ensuring that the dog will pay $1.05 at best.
When I first started betting with online books, I got caught out on a quinella. I liked two dogs at Angle Park and took a quinella for $200. the dogs duly ran 1st and 2nd. The winner paid $7.20, the second dog was showing $6.00 for the win so you would expect the quinella to pay a bit more than $2.70!!! No doubt the book backed the quinella back on the relevant tote.
Practically, you can only bet small with online books otherwise the above will happen. I normally spread the action over the three totes and the three online books.