Does Splitit reserve the full amount on my credit card?

Yes.

The full amount of your purchase is authorized (held) on your credit card to guarantee future payments to your merchant or retailer so you need to have at least that in available credit on your card. This allows us to offer our service to the merchant or retailer without application forms, credit checks, interest or fees for consumers (even if you pay late). You may incur fees or charged from your credit card provider, so you should always check that.

As long as your payment plan is kept up to date, these authorizations are never charged.

The authorization are renewed every 18-21 days and the amount reduces with each monthly payment you make until the balance is cleared. The previous authorization is removed as soon as the new one is obtained.