The schematic diagram is as follows (***4 sheets)

Enter the formula in table G3 in < < product information > >: =IF(B3= "","",D3*F3)? , formula drop-down list.

Enter the formula in << general purchase list >> in F3: =

If (D3= "","",E3 * index (product information! $B$3:$G$ 170, matching (D3, product information! $B$3:$B$ 170，0)，3)？ , formula drop-down list.

Enter the formula = if (D3 = "","",F3 * if ($ D3 = "","",index (product information! $B$3:$G$ 170, matching ($D3, product information! $B$3:$B$ 170，0)，5)))？ , formula drop-down list.

In < < sales report > > in G3, enter the formula: =IF(D3= "","",E3*F3)? , formula drop-down list.

In < < inventory > > enter the formula in B3 cell: =IF(A3= ",0, N(D3)-N(C3)+N(E3))? , formula drop-down list.

Enter the formula = if (is number (match ($ a3, sales report! $ d $3: $ d $100,0)), SUMIF (sales report! $D$3:$D$ 100, $A3, sales report! $E$3:$E$ 100)，" ")？ , formula drop-down list.

Enter the formula = if (or (not (is number (match ($ a3, total receipt! $ d $3: d $100,0)), a3 = """,sumif (total receipt! $D$3:$D$ 100, $A3, total receipt! $F$3:$F$ 100)？ , formula drop-down list.

At this point, the small invoicing system has been established.

Of course, the actual situation is far more complicated than this, and we can completely improve and expand it on this basis. This is another story, not to mention it.