Slice charges one flat fee on the bill amount. The fee is shown to you before you sign — no surprises, no hidden costs. If you pay all your instalments on time, the total you repay is the bill amount plus the fee, and nothing else.
The only other charge that can apply is a late-payment fee if an instalment is missed. The amount is shown in your agreement, and the total cost of credit is capped, so you can never pay more than the cap however late a payment is.
There is no charge to repay early — if you want to settle before the final instalment, the unused part of the fee is refunded.
See also: How do Slice instalments actually work?, Slice glossary: cost of credit and Slice vs a business credit card.