We are developing a PO integration with Non-Inventory items into GP10 using eConnect 10.

Microsoft Dynamics GP\Tools\Setup\Purchasing\Purchase Order Processing\Options.. configuration has
Tax Calculation: Advanced
Tax Options: Base on vendor for Non-inventory items, Freight and Misc

The sample integration XML looks like:

<?xml version="1.0" encoding="UTF-8"?>
<eConnect xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="eConnect.xsd">
  <POPTransactionType>
    <taPoLine_Items>
      <taPoLine>
        <PONUMBER>0014281</PONUMBER>
        <VENDORID>VEN1</VENDORID>
        <LOCNCODE>MAIN</LOCNCODE>
        <ITEMNMBR>ITM1</ITEMNMBR>
        <QUANTITY>3.00</QUANTITY>
        <PRMDATE/>
        <NONINVEN>1</NONINVEN>
        <InventoryAccount>0010-0000-1220  -000</InventoryAccount>
        <ITEMDESC>ITM1 DESC</ITEMDESC>
        <UNITCOST>123.45</UNITCOST>
        <UOFM>Each</UOFM>
        <Purchase_IV_Item_Taxable>3</Purchase_IV_Item_Taxable>
        <Purchase_Site_Tax_Schedu>PURCH</Purchase_Site_Tax_Schedu>
        <ORD>16384</ORD>
        <NOTETEXT/>
       <CURNCYID>CAD</CURNCYID>
      </taPoLine>
    </taPoLine_Items>
    <taPoHdr>
      <PONUMBER>0014281</PONUMBER>
      <VENDORID>VEN1</VENDORID>
      <VENDNAME>VEN1 NAME</VENDNAME>
      <DOCDATE>2009-06-17</DOCDATE>
      <TAXSCHID>PURCH</TAXSCHID>
      <Purchase_Freight_Taxable>3</Purchase_Freight_Taxable>
      <Purchase_Misc_Taxable>3</Purchase_Misc_Taxable>
      <FRTSCHID>PURCH</FRTSCHID>
      <MSCSCHID>PURCH</MSCSCHID>
      <CURNCYID>CAD</CURNCYID>
      <NOTETEXT>TEXT</NOTETEXT>
    </taPoHdr>
  </POPTransactionType>
</eConnect>

Vendor "VEN1" has TaxScheduleID="PURCH"
Vendor "VEN1" has Purchase address, which has TaxScheduleID="PURCH"
SiteID "MAIN" has TaxScheduleID="PURCH"

TaxScheduleID "PURCH" has GST in it, so it' 5%

eConnect 10 creates PO with PO lines no problem. However when I open PO in GP10 client header tax amount nor details tax amount are 

calculated. When I change quantity or cost or Cut/Copy back TaxScheduleID in PO using GP10 client detail taxes get calculated on focus leave 

from the editbox control.

It feels like the tax calculation engine is not being invoked by eConnect 10, because POP10100.TXENGCLD=0 for the integrated PO header.

What am I doing wrong that taxes don't get calculated during the integration? Is there a way to invoke or force tax re-calculation via eConnect 10?

Please advise me what to do.

Not good news... I don't think It'll work the way that you want. The paragraph below is from the help file for a PO transaction. It looks like you'll have to manually supply the tax amount.

 From the help file: 

Header taxes   The calculation that eConnect uses to validate the header tax amount differ between a purchase order and a purchase invoice. To complete your transaction, you must supply the header TAXAMNT value required by a purchase order document.

When eConnect validates the amount of the <taPoHdr> TAXAMNT element, it combines the values from each purchase order line's tax, with related freight tax and miscellaneous tax amounts. If the header TAXAMNT value does not equal this combined tax value, eConnect returns a validation error.

The following example uses a simplified version of a POPTransactionType schema object to illustrate the related elements:

Header.TAXAMNT= 45 (TAXAMNT of 10 + FRTTXAMT of 7 + MSCTXAMT of 28)
Header.FRTTXAMT= 7
Header.MSCTXAMT= 28
Header.UsingHeaderLevelTaxes = 0

Line.TAXAMNT= 10

TaxInsert.FRTTXAMT= 7
TaxInsert.MSCTXAMT= 28
TaxInsert.TAXAMNT= 10