When researching any time from a mandatory vendor - do you ever choose that mandatory vendor even if you will be praying more for an item you can purchase a lot cheaper from a company that is not a mandatory vendor? Example above.
The answer to your question depends on which mandatory source. FAR subpart 13.2 covers actions at or below the micro-purchase threshold (currently $10K for DoD), and FAR 13.201(e) does say that requirements in part 8 apply to purchases at or below the micro-purchase threshold. When there are requirements for supplies, ensure to follow the order of priority listed in FAR 8.002(1).
After verifying the supplies aren't available through existing inventories, the Federal Prison Industry (FPI) is the next mandatory source and has its own unique buying procedures in FAR subpart 8.6 and DFARS subpart 208.6. When buying from FPI, items must be comparable to supplies in the commercial market in terms of price, quality and time of delivery, which will be known after your market research is done. If the supplies are not comparable, then the contracting officer writes a written determination explaining how they are not comparable and you can buy from the commercial marketplace (such as GSA or FAR part 13) using competitive procedures. The written determination is not required if the item is listed in the website referenced in DFARS 208.602-70(b), which means FPI has a significant market share of DoD's market for that particular item.
The next order of priority is AbilityOne IAW FAR subpart 8.7. If the supply item is listed on the Procurement List, then you must buy it from an AbilityOne nonprofit agency, which may be purchased through GSA if there is an existing schedule. Keep in mind if FPI also carries that item that you must buy it from FPI if it is a comparable item.