At a port of entry the applicant pays the TN fee to CBP; by petition the employer pays the Form I-129 fees on the USCIS schedule, with premium processing optional. Consultation and credential-related costs are separate and should be agreed with the firm in writing.
Two routes, two payers
A Canadian engineer can present herself at a land port or airport preclearance with the employer letter and supporting documents and receive a decision the same day, paying the port-of-entry fee set by CBP. Alternatively the U.S. firm can file Form I-129 with USCIS and pay the fees on Form G-1055, which are the employer's responsibility, and can add premium processing for a further fee to get a decision within the published window before she travels.
The petition route gives certainty at the price of time and money; the port route is faster and cheaper but a refusal happens in person. A provincial licence renewal, a credential evaluation if the firm asks for one, and any state licensure the firm wants her to pursue are professional costs the firm may or may not cover. Ask for the reimbursement policy in the offer letter, including whether costs are clawed back if she leaves within a set period, and keep consultation fees separate.