I'm confused about my expenses for work. They're included in my invoices and I got reimbursed for them - do I not declare them in my return?

No. You can't claim expenses that were reimbursed by your employer, because you already have gotten that money back. 

@TurboTaxSusan as a freelancer, I have to invoice these expenses before I get them reimbursed, so they appear as line items on my invoices. I pay GST on the total amount, which is why I’m confused as to why I wouldn’t claim those expenses to offset the corresponding income. 

If you are self-employed, and you billed your clients for the expenses, then yes you can claim the expenses.