US clients, tax situation

Hello everyone.

I'd like to know how I stand with long term USA based client relationships in terms of payment ie: what tax is applicable for payments ranging between three and four figure sums. The work is exclusively 3d modelling and delivery is electronic (via email and/or ftp direct). I don't anticipate breaking the current vat threshold and tbh am not sure how I stand there if I were to do so.

Any pertinent advice would be greatfully recieved with humble thanks.

Jan.