When comparing the revenue number OrderMetrics reports vs. your sales channel, there are a few things to keep in mind.
First, check and make sure that the number of Orders, for the given timeframe and Sales Channel, is correct.
Second, please know that the revenue number calculated by OrderMetrics does not include
Revenue from gift card transactions
For example, you may see $1,000 in revenue reported by Shopify for a given month. However OrderMetrics may report $950 for the same month because $50 of that $1,000 was gift card revenue. The discrepancy can usually be explained by the type of report you are running in your sales channel, and whether that report includes or excludes gift card revenue, refunds, and discounts.