QB Issue Resolution:

QB keeps average cost for inventory, unless you have enterprise and have turned on FIFO

stock the items, who you order from is immaterial, then sell them

I’m not sure what 10 PCs by 5 means, or
15 items with 7 USD

QB keeps inventory value in the base currency if you use multicurrency, regardless of what currency you pay for it in, that amount is converted to your base currency and stocked

Resolution for Issue 'Adding inventory – same product, different cost different PO same vendor' available: Yes (Solved).
Source: Intuit Community forum.

Adding inventory – same product, different cost different PO same vendor: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for cost
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Error Recovery for cost [Canada]
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