I just wanted to know if the 45.99 for the smart box is a one time payment. Thanks
The £45.99 payment is the purchase for the box only which you will own out right.
If you wish to watch NowTV content on the box you will either need to subscribe to monthly NowTV Passes which are extra or look at NowTV Gift Card Vouchers which work out cheaper.
More information here about the NowTV service on how it works.
If you are interested only on the third party Apps on the box, then you can use these Apps without subscribing to NowTV where there is more information in this link below.
Look around for NowTV authorised Retailers where you might be able to purchase the box cheaper.
Here's one example link below for a better deal than buying directly from the NowTV website.
Also please note that the NowTV App on the 4K NowTV box is only currently 720p and the only 4K Apps on the box is Netflix with a UHD Sub, BBC iPlayer & YouTube content permitted.