Wealthsimple Trade charges a currency conversion fee of 1.5% on top of the corporate rate on all trades for US-denominated assets (both buys and sells). The fee is encompassed in the total foreign exchange (FX) rate charged, which is shown on the estimate before you submit your order as well as on the trade confirmation after the order has been filled.
As an example, if you were to purchase $100 USD worth of a US-denominated asset, assuming for illustrative purposes a 1.3 corporate rate (note that this rate fluctuates) for converting CAD to USD, the total (before fees) would cost you $130 CAD. Including the currency conversion fee, the total FX rate would be 1.3195, bringing the total amount for the purchase to $131.95 CAD, with the $1.95 spread being the currency conversion fee in this case.
If you were to sell $100 USD worth of assets, again assuming a 1.3 corporate rate (for illustrative purposes) for converting USD to CAD, you would expect to receive $130 CAD in proceeds from the sale without factoring in the fee. Including the currency conversion fee, the total FX rate would be 1.2805 on Wealthsimple Trade, and you would receive $128.05 in total from the sale with the $1.95 spread being the fee. All told, you would be paying $3.90 CAD in currency conversion fees for this roundtrip trade.